stronomy 


Hin'£fs  £  ;e  Should  Knov 

jttlie  Sun..  Moon.  and  Stan 


THE    PLEIADES 

.4   long  exposure  photograph  showing  the  wonderful  nebulosity  enveloping  this  whole  group 

Viewed  with  the  eye  these  stars  here  shown  drowned  in  foggy  light  shine  clear  and  brilliant 

From  a  photograph  taken  at  the  Yerkes  Observatory 


The   Essence  of 
Astronomy 

Things  Every  One  Should  Know  about 
the  Sun,  Moon,  and  Stars 

By 
Edward  W.  Price 


Illustrated 


G.  P.  Putnam's  Sons 
New  York  and  London 
fmfcfcerbocfcer  press 


COPYRIGHT,  1914 

BY 
G.  P.  PUTNAM'S  SONS 


,  flew 


So 

MY  "ASTRONOMICAL  WIDOW" 


292213 


PREFACE 

THIS  small  volume  is  not  offered  to  the  public 
as  an  original  treatise  upon  its  subject.  It  is 
merely  a  brief  compilation  of  the  facts  of  Astron- 
omy, as  presented  by  the  acknowledged  au- 
thorities, together  with  some  slight  reference  to 
various  theories  not  yet  substantiated. 

All  technical  terms,  and  all  mathematics, 
except  certain  simple  figures  necessary  for  de- 
scription and  comparison,  have  been  omitted, 
and  no  signs  or  symbols  have  been  used  except 
those  to  be  found  in  the  ordinary  almanacs. 
These  last  do  not  appear  in  the  text,  but  are 
given  on  a  separate  page  in  the  back  part  of  the 
volume.  In  spite  of  these  omissions,  it  is 
hoped  that  this  compilation  may  have  some 
value  as  a  concise  reference  book  for  the  general 
reader  or  for  schools.  With  a  view  toward 
simplicity,  the  writer  has  given  no  exact  figures 
such  as  would  be  necessary  in  an  advanced 
textbook,  but  has  stated  all  numerical  values  in 
the  closest  approximate  terms. 


vi  Preface 

There  has  been  added — what  the  compiler  has 
never  happened  to  see  before — a  tabulated 
chronology  of  the  main  events  in  the  history  of 
Astronomy,  and  also  a  chapter  very  briefly  de- 
scribing the  various  instruments  now  used  in 
the  great  observatories. 

In  the  bibliography  at  the  end  of  the  volume, 
are  listed  the  titles  of  all  the  books  used  for 
reference  in  checking  the  descriptions,  figures, 
and  statements  given.  To  the  authors  of  all 
of  these  books,  the  writer  is  much  indebted. 
Following  each  title  is  a  short  note,  endeavoring 
to  present  an  idea  of  the  scope  of  that  volume. 
If,  upon  reaching  this  bibliography,  the  reader  is 
enough  interested  in  the  greatest  and  most 
ancient  of  sciences  to  turn  to  some  of  these  real 
books  upon  the  subject,  the  main  purpose  of 
these  pages  will  have  been  accomplished. 

It  will  be  noticed  that  the  greater  part  of  the 
volume  is  devoted  to  the  Solar  System,  and  that 
to  the  Universe  as  a  whole  is  given  but  scant 
space.  It  is  about  the  Solar  System  that  the 
astronomical  facts  swarm  thickest,  while,  though 
a  marvelous  amount  is  really  known  about  the 
vast  stellar  hordes,  it  is  often  difficult  to  sepa- 


Preface 


vn 


rate  fact  from  logical  surmise.  The  more  essen- 
tial of  the  stellar  facts,  and  many  of  the  generally 
accepted  and  most  logical  of  those  theories  which 
should  interest  the  general  reader,  are,  however, 
given. 

The  grouping,  or  apparent  grouping,  of  the 
stars  into  constellations  has  not  been  touched 
upon.  It  is  not  possible  to  give  in  short  space 
enough  description  of  them  to  make  their  posi- 
tions clear.  The  bibliography  refers  to  several 
works  admirably  written  for  just  that  purpose. 

Nor  has  it  been  possible  to  find  room  for  any  of 
the  great  discussions  concerning  the  beginnings 
of  the  Solar  System.  In  passing,  however,  it 
may  be  said  here  that  several  theories,  and 
attractively  logical  ones,  have  been  advanced  in 
the  past  few  years  since  the  rejection  by  most 
astronomers  of  the  Nebular  Hypothesis  as 
conceived  by  Laplace. 

E.  W.  P. 

NEW  YORK 
February,  1914. 


CONTENTS 

CHAPTER  PAGE 

I    The  SOLAR  SYSTEM    .  .         .        i 

II    THE  SUN           .        .  .         .       10 

III  MERCURY         :....      .  .         .       19 

IV  VENUS      .        *.         .  «         .      25 
V    THE  EARTH       .         .  > .         .      30 

VI    THE  MOON        .         .  .         .      38 

VII    MARS 48 

VIII    THE  ASTEROIDS          .  .        ...      57 

IX    JUPITER    .....       61 

X    SATURN    .         .         .  .         .      68 

XI    URANUS    .         .         ,  .         .77 

XII    NEPTUNE          .  .         .82 

XIII  COMETS    .         .         .  .         .88 

XIV  METEORS          .         .  ...     100 
XV    ECLIPSES           .         .  .         .     106 

XVI    THE  "FIXED  STARS".  ...     113 

ix 


x  Contents 

CHAPTER  PAGE 

XVII    NEBULA  .         .        ...         .     128 

XVIII    FREAKS  AND  ODDITIES  OF  THE 

SKIES         ...        .     138 

XIX    ASTRONOMICAL  INSTRUMENTS       .     158 
XX    CHRONOLOGY     .         .         .         .184 

ASTRONOMICAL  SIGNS  AND  SYM- 
BOLS .         .         .         .     197 

BIBLIOGRAPHY  .        .        .        .198 


ILLUSTRATIONS 


THE  PLEIADES    .         .         ,         Frontispiece 

A  long-exposure  photograph  showing  the 
wonderful  nebulosity  enveloping  this  whole 
group. 

From  a  photograph  taken  at  the  Yerkes 
Observatory. 

THE  SUN       .         .         .  .12 

Showing  a  group  of  Sun-spots. 
From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

SOLAR  PROMINENCES       .         .         .       .„.      16 

Two  photographs  showing  the  vast  changes 
occurring  in  ten  minutes.  October  10,  1910. 

From  photographs  taken  at  the  Yerkes  Ob- 
servatory. 

THE  MOON,  AT  NINE  AND  THREE-QUARTER 

DAYS      .         .         .         .         .        "..      46 
From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

MARS    .         .         .         .        .        «         .      54 

From  four  drawings  by  Professor  Lowell. 
JUPITER,  1910         .  \         .         .64 

From  a  photograph  taken  at  the  Lowell  Ob- 
servatory. 


xii  Illustrations 


SATURN         »         .         .         .      .  .         .      70 

December  23,  1912.  Showing  the  dark  cap, 
the  belts,  the  divisions  in  the  rings,  and  the 
oval  form  of  the  planet. 

From  a  photograph  by  E.  C.  Slipher,  taken 
at  the  Lowell  Observatory. 

COMET  MOREHOUSE,  1908       .         .       •',      90 

Two  photographs  showing  the  rapid  changes 

in  the  tail. 
Photographs  from   the  Yerkes   Observatory; 

taken  three  hours  apart. 

HALLEY'S  COMET  .         ,         .         .         .      96 

Photographed  May  13,  1910.  Showing  pe- 
culiar shape  of  tail  and  a  meteor  trail  cross- 
ing it.  Length  of  tail  50°. 

From  a  photograph  taken  at  the  Lowell  Ob- 
servatory. 

SOUTHERN  REGION  OF  ORION  .         .         .104 

Showing  position  of  the  great  nebula,  and  a 
meteor  trail. 

From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

LARGER  STARS  OF  THE  PLEIADES      .         .     118 

From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

NEBULA  (N.  G.  C.  6992)  IN  CYGNUS         .     132 

A  wonderful  example  of  an  irregular  nebula, 
showing  very  faint  lace-like  appearance. 

From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 


Illustrations  xiii 

PAGE 

THE   "WHIRLPOOL"    NEBULA   IN   CANES 

VENATICI        .         •         •         •  ^     •     132 

From  a  photograph  taken  at  the  Lick  Ob- 
servatory. 

THE  GREAT  NEBULA  IN  ORION         .         .136 

The  best  known  example  of  the  gaseous  nebula. 
From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

THE  MILKY  WAY  ABOUT  CHI  CYGNI         .     140 

A  splendid  example  of  a  Star  Cloud  is  shown 
at  the  center  of  the  picture. 

From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

THE  DOUBLE  STAR  SWARM  IN  PERSEUS      .     144 
From  a  photograph  taken  at  the  Yerkes  Ob- 
servatory. 

THE  GREAT  SOUTHERN  STAR-CLUSTER  o> 

CENTAURI       .         .         .         .         .     160 

One  of  the  best  two  examples  of  a  true  star- 
cluster. 

Photographed  by  S.  I.  Bailey  at  the  South 
American  Station  of  Harvard  Observatory. 

DIAGRAMS  IN  TEXT 

THE  ORBITS  OF  THE  TERRESTRIAL  PLANETS  4 
THE  ORBITS  OF  THE  MAJOR  PLANETS  .  5 
THE  PHASES  OF  THE  MOON  .  .  ~  ,  42 


XIV 


Illustrations 


PAGE 

THE  EQUATORIAL  MOUNTING  OF  A  TELE- 
SCOPE    .         .         ,         .         .         .     170 


It  is  a  pleasure  to  acknowledge  the  permission 
to  reproduce  the  above  photographs  so  kindly 
extended  by  the  Directors  of  the  Yerkes  Ob- 
servatory, the  Lick  Observatory,  the  Harvard 
College  Observatory,  and  the  Lowell  Observa- 
tory. 

To  them,  and  to  Professor  Lowell  in  particular, 
for  his  interest,  trouble,  and  great  courtesy,  the 
author  wishes  to  express  his  gratitude. 


The  Essence  of  Astronomy 


The  Essence  of  Astronomy 


CHAPTER  I 

THE  SOLAR  SYSTEM 

RUSHING  through  space,  bound  whither  we 
know  not,  save  for  the  mere  direction,  speeds  the 
vast  globe  called  the  Sun,  at  a  rate  of  nearly  12 
miles  per  second.  Around  this  falling  body — and 
a  true  and  stupendous  fall  it  is — an  almost  infinite 
number  of  smaller  bodies,  ranging  from  tiny 
particles  to  great  masses  thousands  of  miles  in 
diameter,  revolve  with  a  marvelous  regularity 
and  order.  This  is  the  Solar  System.  Although 
almost  inconceivable  in  size,  its  huge  dimensions 
shrink  to  microscopic  estimates  when  we  meas- 
ure on  its  every  side  the  unbelievable  depths  of 
space  which  bound  its  isolation.  For  countless 
centuries,  the  Sun  and  its  family  have  been 
approaching,  at  a  speed  of  over  a  million  miles 
i 


2        THe  Essence  of  .Astronomy 

a  day,  a  definite  section  of  the  Universe,  and  yet 
it  seems  no  closer  than  thousands  of  years  ago. 
In  truth  the  Solar  System  is  but  a  cluster  of  tiny 
motes  dancing  in  the  infinite. 

According  to  our  present  knowledge,  the 
Solar  System,  measures  over  5,000,000,000  of 
miles  across,  and  these  boundaries  are  decided 
only  by  the  "stay-at-home"  members  of  the 
family,  the  planets.  How  far  those  eccentric 
wanderers,  the  comets,  may  extend  these  limits, 
we  cannot  tell. 

In  the  center  of  this  great  system,  spins  the 
Sun,  a  huge  globular  mass  of  flaming  gases, 
inconceivably  hot,  giving  forth  practically  all  the 
light  and  heat  for  its  whole  family.  This  Sun 
is  a  star,  as  much  so  as  any  of  the  thousands  we 
see  at  night  twinkling  in  the  skies,  showing  from 
their  incalculable  distances  only  as  tiny  points 
of  light;  the  humiliating  truth  being  that  our 
Sun  is,  indeed,  but  one  of  the  smaller  stars,  and 
it  is  only  its  nearness  which  blinds  us  to  its  true 
proportions. 

Around  itself,  the  Sun,  by  the  mighty  power  of 
gravitation,  swings,  in  the  order  named,  the 
following  eight  bodies  called  planets:  Mercury, 
Venus,  the  Earth,  Mars,  Jupiter,  Saturn,  Ura- 


THe  Solar  System  3 

nus,  and  Neptune,  which  all  revolve  in  slightly 
elliptical  orbits.  The  orbits  of  the  planets  lie 
approximately  in  the  same  plane  (the  plane  of 
the  ecliptic,  as  it  is  named),  so  that  the  shape  of 
the  Solar  System  as  a  whole  is  that  of  a  great 
flat  disk.  These  planets  all  shine  only  by  re- 
flecting a  portion  of  the  Sun's  light.  They  have 
no  light  of  their  own.  Their  brilliancy  is  of  wide 
range.  This  is  mainly  because  of  the  difference 
in  their  distances  from  us,  but  is  also  due,  in 
part,  to  the  difference  in  their  sizes  and  reflective 
power  (albedo),  as  well  as  to  their  position  in 
relation  to  the  Sun  and  the  Earth.  Because  of 
distance,  Uranus  is  but  barely  visible  to  the 
naked  eye,  and  Neptune  is  never  seen  without 
a  telescope;  while  Mercury  is  so  close  to  the 
Sun  that  it  appears  but  at  certain  intervals,  and 
then  under  perfect  conditions  only  is  visible 
without  a  glass.  Four  of  the  planets,  however, 
Venus,  Mars,  Jupiter,  and  Saturn,  are  those 
"stars"  which  shine  to  the  naked  eye  clear  and 
bright,  without  the  "twinkle"  of  the  real  stars, 
and  two  of  them,  Venus  and  Jupiter,  are  those 
which  so  often  sparkle  in  our  early  evening  and 
morning  skies  with  such  luster  and  brilliancy. 
The  inner  four  of  the  eight  planets  are  com- 


4       THe  Essence  of  Astronomy 

paratively  small  and  dense,  or  heavy,  and  the 
outer  four  are  large  and  light. 

The  periods  of  their  revolutions  about  the 


THE   ORBITS   OF  THE   TERRESTRIAL   PLANETS 
AND    ONE   OF  THE    PLANETOIDS 

(Drawn  approximately  to  scale) 


Sun — their  "years," — vary  with  their  distances 
from  it,  between  88  days  and  164  years.  This 
is  in  obedience  to  a  definite  law  of  motion  which 


THe  Solar  System  5 

insists  upon  an  exact  relation  between  such  times 
and  distances. 

The  orbits  of  the  planets  in  regard  to  the  Sun 


THE    ORBITS    OF  THE    MAJOR    PLANETS 

(Drawn  approximately  to  scale) 


are  ellipses,  it  is  true,  but  in  relation  to  space 
they  are,  because  of  the  motion  of  the  Sun  along 
its  celestial  path,  vast  spirals;  so  that,  far  from 


6        XKe  Essence  of  Astronomy 

returning  to  the  same  spot  in  the  Universe  at 
the  completion  of  one  revolution  about  the  Sun, 
a  planet  never  passes  over  the  same  place  twice. 

Around  most  of  these  planets,  revolve  from 
one  to  ten  smaller  bodies,  called  moons  or  satel- 
lites. These  satellites,  of  which  our  own  Moon 
is  one,  shine  also  only  by  reflected  light. 

The  Sun,  planets,  and  satellites  all  rotate  upon 
their  axes,  in  periods  ranging  from  a  few  hours 
to  several  months. 

Between  the  orbits  of  Mars  and  Jupiter  is  a 
large  number  of  still  smaller  bodies,  called  as- 
teroids or  planetoids.  Of  these,  about  800  have 
been  discovered,  sweeping,  each  in  its  own  orbit, 
as  part  of  a  great  ring  around  the  Sun. 

Most  unique  of  the  solar  family  are  the 
"fiery-haired"  comets,  those  ancient  portents 
of  evil,  which  swing  in  narrow  ellipses  of  great 
variety,  some  contentedly  remaining  within  the 
planetary  boundaries,  and  others  rushing  far 
into  the  emptiness  beyond  to  return  again  to 
view  only  after  hundreds  and  even  thousands  of 
years.  Indeed,  some  astronomers  believe  that  a 
large  percentage  of  the  comets  are  truly  wan- 
derers in  space,  which  bow  to  the  Sun  but  once, 
and  pass  on  never  to  return. 


THe  Solar  System  7 

Least  in  size,  but  by  far  the  most  numerous, 
are  the  meteors,  chips,  so-to-speak,  from  worlds 
in  the  making,  some  of  which  are  herded  to- 
gether in  vast  swarms,  while  others  pursue  more 
lonely  paths.  But  all,  alike,  obey  the  great  law, 
and  revolve  in  stated  orbits  around  the  Sun, 
until  they  collide  with  one  of  the  planets,  or, 
disturbed  by  a  too  close  approach  to  such  a  large 
body,  are  swung  into  a  new  path  that  ends  in 
the  Sun  itself.  It  is  the  fiery  dissipation  of  a 
meteor,  heated  to  a  great  temperature  by  the 
friction  of  our  atmosphere,  which  causes  the 
phenomenon  of  a  "shooting-star." 

It  is  probable,  also,  that  a  "cloud"  of  almost 
dust-like  particles  of  matter  extends  through 
much  of  the  Solar  System,  particles  too  small  to 
be  of  appreciable  size  or  weight.  The  existence 
of  these  is  strongly  suspected  by  reason,  and  is 
believed  to  be  demonstrated  by  two  phenomena, 
later  described  in  Chapter  XVII.,  the  Zodiacal 
Light,  and  the  Gegenschein. 

There  was  for  some  time  believed  to  be  a 
planet  body  revolving  about  the  Sun  within  the 
orbit  of  Mercury.  This  hypothetical  planet  was 
even  given  a  name — Vulcan.  Its  existence  has 
now,  however,  been  absolutely  disproved. 


8       TKe  Essence  of  Astronomy 

Also,  far  beyond  Neptune,  another  planet  is 
supposed  to  creep  in  its  slow  path.  A  sys- 
tematic search  for  this  has  extended  over  many 
years,  but  has  as  yet  been  unsuccessful.  If  such 
a  planet  does  exist,  as  seems  likely,  it  must  be  at 
a  vast  distance,  over  four  billions  of  miles,  and 
probably  is  invisible  even  with  the  aid  of  our 
greatest  telescopes.  If  it  is  found,  it  will  be 
the  photographic  plate  which  "sees"  it,  and 
which  will  record  its  lazy  motion  across  the 
skies. 

A  scale  which  helps  one  to  remember  the  pro- 
portions of  the  Solar  System  is  the  following: 


The  Sun— a  globe  2  feet 
in  diameter 

Mercury — a       mustard- 
seed  55  yards  away  from  the  globe 

Venus — a  small  pea  95     " 

The  Earth— a  large  pea          143     "        "        "      "      " 

Mars — the  head  of    a 

rather  large  pin  215     "        "        "      "      " 

The   Asteroids — grains 

of  sand  220-270    " 

Jupiter — a  large  orange          740     "        " 

Saturn — a  small  orange  nearly  a  mile  away 

Uranus — a  marble  over  a  mile  and  a  half 

away 

Neptune — a  larger  marble  over  two  and  a  half  miles 

away 


THe  Solar  System  9 

All  the  members  of  the  solar  family  will  be 
taken  up  chapter  by  chapter,  and  it  seems  fitting 
to  begin  with  the  Sun  itself. 


CHAPTER  II 

THE  SUN 

THE  Sun  is  the  central  body  of  the  Solar  Sys- 
tem, and  is  far  larger  than  all  the  other  members 
of  the  system  put  together. 

Its  diameter  is  about  866,400  miles.  Its  sur- 
face is  nearly  12,000  times  that  of  the  Earth,  and 
its  volume,  or  bulk,  nearly  1,300,000  times  as 
much  as  that  of  our  planet. 

Its  mean,  or  average,  distance  from  the  Earth 
is  not  quite  93,000,000  miles. 

The  Sun  rotates  upon  its  axis  in  a  mean  time 
of  25  days,  7  hours,  48  minutes.  The  equatorial 
regions,  or  middle  part,  of  the  Sun  turn  con- 
siderably faster  than  the  poles.  That  is,  a  point 
on  the  Sun's  equator  actually  completes  a  full 
rotation  sooner  than  a  point  in  a  higher  latitude, 
each  particle  nearer  the  equator  slipping  by  its 
neighbor  nearer  the  pole.  This,  which  cannot  hap- 
pen in  the  rotation  of  a  rigid  body  like  the  Earth, 
is  possible  because  of  the  Sun's  lack  of  solidity. 

JO 


The  Sun  II 

Its  axis  is  inclined  to  the  ecliptic,  as  the  plane 
of  the  Earth's  revolution  about  the  Sun  is  called, 
about  seven  degrees. 

The  Sun  is  moving  through  space  in  the 
direction  of  the  bright  star  Vega  at  a  rate  of 
between  n  and  12  miles  a  second. 

The  mass,  or  the  amount  of  matter,  of  the  Sun 
is  about  330,000  times  that  of  the  Earth;  its 
density,  or  the  compactness  of  the  matter,  is, 
however,  only  about  one  quarter.  Its  attraction 
for  bodies  at  its  surface  is  nearly  28  times  that  of 
the  Earth.  A  body  weighing  here  200  pounds 
would  on  the  Sun  weigh  nearly  three  tons. 

The  Sun  has  a  dense  atmosphere,  quite  dif- 
ferent, however,  from  our  own. 

The  temperature  of  the  Sun  is  very  difficult 
to  measure.  It  is,  according  to  the  best  authori- 
ties, about  12,000  degrees  Fahrenheit. 

The  brilliance  of  its  light  is  also  very  difficult 
to  estimate.  This,  at  its  surface,  must  be  at 
least  150  times  that  of  a  calcium  light,  one  of  the 
brightest  lights  that  we  can  produce  artificially. 

Of  the  light  and  heat  of  the  Sun,  the  Earth 
intercepts  but  a.aoo.oWooo  part. 

Through  a  telescope,  the  surface  of  the  Sun 
shows  a  strangely  mottled  effect,  with  here  and 


12      TKe  Essence  of  Astronomy 

there  a  dash  of  more  brilliant  light.  From  time 
to  time,  there  appear,  also,  great  black-looking 
spots,  these  seeming  black  only  by  contrast  to  the 
rest  of  the  surface.  These  are  the  so-called 
Sun-spots,  and  seem  to  be  vast  cyclonic  holes 
in  the  superficial  gaseous  layer,  or  atmosphere, 
surrounding  the  main  body  of  the  Sun.  The 
frequency  of  these  spots  varies  in  a  cycle  of  about 
eleven  years,  the  reason  for  which  is  not  known. 
These  spots  also  never  appear  near  the  poles  of 
the  Sun,  but  at  the  beginning  of  a  cycle  show 
in  the  middle  latitudes,  and  subsequently  break 
out  nearer  and  nearer  the  equator,  close  to  which 
they  are  present  during  the  period  of  greatest 
frequency.  The  size  of  the  spots  is  of  great 
range.  Some  are  comparatively  small,  and  are 
visible  only  through  the  great  telescopes,  while 
others  have  been  measured  well  over  100,000 
miles  in  diameter,  and  have  been  easily  seen 
with  the  naked  eye.  The  duration  of  the  spots 
also  differs  much.  Some  last  for  a  few  hours  only 
while  others  appear,  with  but  slight  changes, 
for  two  or  three  months.  It  was  by  observation 
of  the  spots  that  the  time  of  the  Sun's  rotation 
was  found.  Incidentally,  it  is  interesting  to 
know  that  when  Galileo  first  announced  his 


THE    SUN 

Showing  a  group  of  sun-spots.     Note  in  the  spots  the  dark  center 
(the  umbra)  and  the  lighter  border  (the  penumbra) 
From  a  photograph  taken  at  the  Yerkes  Observatory 


TKe  Sun  13 

discovery  of  the  spots,  his  statement  was  met 
with  scornful  derision.  As  if  there  could  be 
blemishes  upon  the  face  of  our  great  luminary! 

During  a  total  eclipse  of  the  Sun,  when  the 
blinding  light  of  the  globe  itself  is  cut  off,  there 
are  seen  to  shoot  forth  great  sprays  and  tongues 
of  flame,  to  the  height  of  sometimes  over  300,000 
miles.  For  many  years  these  were  supposed  to 
be  appendages  of  the  Moon,  and  in  consequence 
to  be  comparatively  small,  but  they  have  long 
since  been  proven  to  belong  to  the  Sun.  These 
appalling  flames  lash  out  with  a  speed  of  often 
several  hundred  miles  a  second,  and  bespeak  an 
explosive  force  behind  them  almost  inconceiv- 
able. These  eruptions,  or  prominences  as  they 
are  called,  undoubtedly  occur  at  all  times  over 
most  of  the  Sun's  surface.  We  can  see,  however, 
only  those  which  extend  beyond  the  edge  of  the 
apparent  disk.  These  prominences  can  now  be 
studied  without  waiting  for  an  eclipse  by  means 
of  the  instrument  known  as  the  spectroscope. 
There  are  also  to  be  seen  great  cloud-like  masses 
floating  high  above  the  surface  of  the  Sun  as  do 
clouds  in  our  atmosphere.  These  are  literally 
clouds  of  vapors,  for  the  most  part  hydrogen, 
helium,  and  calcium.  They  are  sometimes  called 


14      THe  Essence  of  Astronomy 

"floating  prominences. "  They  last  much  longer 
than  the  eruptive  prominences. 

Another  prominent  feature  of  the  Sun,  seen 
only  at  a  total  eclipse,  however,  is  the  corona. 
This  is  a  wonderfully  soft  radiance  which  extends 
often  for  millions  of  miles,  in  great  curving 
wings  and  streamers,  and  always  shows  as  a 
glorious  halo  around  the  black  center  of  the 
eclipse.  Those  who  have  been  so  fortunate  as  to 
have  seen  a  total  eclipse  speak  of  the  corona  as 
a  most  marvelously  beautiful  sight.  The  nature 
of  the  corona  is  not  known.  Under  the  analysis 
of  the  spectroscope,  it  shows  a  substance  which, 
as  yet,  has  not  been  found  on  the  Earth.  This 
has  been  called  coronium. 

There  has  been  much  speculation  as  to  the 
means  by  which  the  Sun  maintains  its  great 
temperature  when  it  continues  radiating  such 
enormous  quantities  of  heat.  It  is  easily  demon- 
strated that  no  combustion,  or  burning,  as  we 
know  such,  can  account  for  this.  The  geologists 
demand  millions  of  years  as  necessary  to  account 
for  the  condition  of  the  Earth ;  and  the  Sun  must 
be  older  than  the  Earth.  Grant  any  such  age, 
and,  under  the  combustion  theory,  the  Sun 
would  have  been  a  dead  cinder  long  ago. 


THe  Sun  15 

While  still  believing  in  the  contraction  theory, 
astronomers  are  now  inclined  to  admit  the  pos- 
sibility of  a  radio-active  condition  which  may 
have  a  much  greater  effect  than  is  supposed. 

The  contraction  theory  is  based  upon  the 
fact  that,  given  a  gaseous  mass  of  matter  the 
size  of  the  Sun  at  present,  heated  to  a  high 
temperature,  this  mass,  by  contracting  in  volume, 
that  is  by  falling  together  in  consequence  of 
gravitational  pull,  less  than  a  mile  in  diameter 
per  year,  or  866.looo  of  its  diameter,  could  radiate 
the  amount  of  heat  which  the  Sun  does  now,  and 
still  have  enough  left  to  keep  the  remaining  smaller 
bulk  of  itself  at  the  original  temperature,  or  even 
to  raise  the  temperature  slightly  higher.  In  other 
words,  the  Sun,  although  losing  a  great  amount  of 
heat,  may  nevertheless  be  growing  hotter  day  by 
day.  This  sounds  paradoxical  for  the  moment, 
but  a  consideration  of  the  difference  between 
amount  of  heat  and  temperature  will  make  it  clear. 

Because  of  a  curious  optical  illusion,  it  is  not 
possible  to  measure  with  great  accuracy  any 
very  brilliant  object.  For  this  reason,  we  cannot 
tell  yet  whether  the  Sun  is  really  decreasing  in 
size.  A  decrease  of  fifty  miles  in  its  diameter 
would  not  be  noticeable. 


1 6     TKe  Essence  of  Astronomy 

The  Sun  also  acquires  a  certain  amount  of  heat 
by  the  impact  of  meteors,  which  doubtless  fall 
in  great  numbers  to  its  surface.  As,  however,  the 
fall  into  the  Sun  of  a  body  as  large  as  the  Earth 
would  produce  heat  enough  to  last  only  for  a  com- 
paratively short  time,  it  seems  hardly  possible 
that  the  fall  of  meteors  can  be  considered  as  a  real 
factor  in  the  continuation  of  the  Sun's  store. 

Although  the  Sun  is  but  a  small  star,  it  is, 
nevertheless,  to  us  the  most  important  of  all 
stars.  We  might  dispense  with  a  great  propor- 
tion of  the  light  received  from  this  enormous 
body,  but  without  its  heat  we  should  have  no 
existence.  It  is  the  heat  of  the  Sun  which 
stimulates  our  vegetation,  and  vegetation  sup- 
ports directly  or  indirectly  all  animal  life.  It  is 
the  heat  of  the  Sun  which  causes  all  atmospheric 
disturbances,  and  in  consequence  keeps  our  air 
constantly  freshened.  It  is  the  heat  of  the  Sun 
which  causes  water  evaporation  and  the  result- 
ant rains,  maintaining  the  purity  of  our  water 
supplies.  In  fact,  it  is  the  heat  of  the  Sun  which 
is  primarily  responsible  for  all  the  changes  ne- 
cessary to  make  the  Earth  habitable.  From  it  we 
derive  all  our  sustenance  as  well  as  all  forms  of 
mechanical  power. 


The  Sun  17 

The  proportions  and  properties  of  the  Sun  are 
so  vast  that  it  is  practically  impossible  to  grasp 
them  without  some  comparison  with  familiar 
standards.  A  few  of  these  are  given  below. 

If  the  Earth  were  placed  at  the  center  of  the 
Sun,  the  Moon  revolving  round  the  Earth  at  its 
present  distance  would  reach  only  a  little  more 
than  half  way  to  the  surface  of  the  Sun.  There 
would  still  be  over  190,000  miles  between  the 
Moon  and  the  surface.  And  yet,  a  train  running 
a  mile  a  minute  would  take  three  and  one  half 
months  to  cover  a  distance  corresponding  to 
that  which  separates  the  Moon  from  the  Earth. 

This  same  train,  which  would  go  round  the 
Earth  in  seventeen  days,  would  require  five 
years  to  make  the  circuit  of  the  Sun. 

If  the  Sun  were  a  flat  disk,  as  it  appears,  109 
globes  as  large  as  the  Earth,  touching  edge  to 
edge,  would  be  necessary  to  reach  across  it. 

We  receive  from  the  Sun  about  600,000  times 
as  much  light  as  we  get  from  the  Moon.  Were 
the  sky  covered  with  full  moons  it  would  not 
approach  the  radiancy  of  daylight. 

The  heat  of  the  Sun,  to  quote  the  late  Prof. 
Young,  whose  estimate  of  the  temperature  was 
given  above,  may  be  illustrated  by  the  fact  that 


1 8      THe  Essence  of  Astronomy 

"if  a  bridge  of  ice  could  be  formed  from  the 
Earth  to  the  Sun  by  a  column  of  ice,  2.1  miles 
square  and  93,000,000  miles  long,  and  if  in  some 
way  the  entire  solar  radiation  could  be  concen- 
trated upon  it,  it  would  be  melted  in  one  second, 
and  in  seven  more  would  be  dissipated  in  vapor. " 


CHAPTER  III 

MERCURY 

MERCURY  is  the  smallest  of  the  bodies  called 
planets,  and  revolves  nearer  to  the  Sun  than  any 
of  the  others. 

Its  diameter  is  about  3030  miles,  nearly  half 
as  much  again  as  that  of  our  Moon.  Its  surface 
is  about  one  seventh  that  of  the  Earth,  and  its 
volume  about  one  eighteenth. 

Its  mean,  or  average,  distance  from  the  Sun 
is  36,000,000  miles.  Its  elliptical  orbit  is  more 
eccentric,  or  elongated,  than  those  of  the  other 
planets,  and  its  distance  from  the  Sun  varies 
between  43,500,000  miles  and  28,500,000  miles. 

Its  distance  from  the  Earth  at  its  closest 
approach  is  about  50,000,000  miles,  and  when 
farthest  from  it,  about  136,000,000  miles. 

Mercury  revolves  around  the  Sun  in  very 
nearly  88  days.  This  is  the  "year"  of  Mercury. 

It  rotates  upon  its  axis  in  the  same  time,  88 
days,  and  in  consequence  always  turns  the  same 
19 


so     THe  Essence  of  Astronomy 

side  toward  the  Sun,  this  side  being  in  perpetual 
daylight,  the  other  in  perpetual  night. 

Its  axis  is  perpendicular  to  the  plane  of  its 
orbit  about  the  Sun.  There  are  upon  Mercury, 
therefore,  no  seasons  such  as  we  experience  on 
the  Earth. 

It  moves  in  its  path  of  revolution  at  a  mean 
speed  of  about  29  miles  per  second.  This  speed 
varies,  however,  between  about  36  miles  per 
second,  when  nearest  the  Sun,  and  about  23 
miles  per  second,  when  farthest  away. 

The  mass  of  Mercury  is  not  definitely  known. 
According  to  the  latest  measurements,  the  den- 
sity of  the  planet  is  somewhat  less  than  that  of 
the  Earth.  The  weight  of  Mercury  is,  therefore, 
slightly  less  than  five  and  one  half  times  that  of 
a  globe  of  water  the  same  size.  If  these  estimates 
are  correct,  as  seems  probable,  its  attraction  for 
bodies  at  its  surface  is  about  one  half  that  of  the 
Earth's.  That  is,  a  body  weighing  200  pounds 
here  would  weigh  less  than  100  pounds  there. 

Mercury  has  practically  no  atmosphere. 

It  receives  from  the  Sun  on  an  average  nearly 
seven  times  as  much  light  and  heat  as  does  the 
Earth;  when  nearest  the  Sun  (or  at  perihelion, 
as  this  point  of  a  planet's  orbit  is  called)  over 


Mercury  21 

nine  times  as  much,  and  when  farthest  from  the 
Sun  (aphelion)  over  four  times  as  much.  This 
variation  in  the  heat  received  does,  therefore, 
produce  a  sort  of  change  of  seasons,  and  a  very 
severe  one. 

Mercury  reflects  back  from  its  surface  about 
14  per  cent,  of  the  light  received. 

Viewed  with  a  telescope,  Mercury  shows 
"phases,"  as  does  our  Moon  (see  Chapter  VI.). 
In  passing  from  "new"  to  "full"  and  back  again 
about  115  days  are  consumed.  If  the  Earth 
were  stationary,  the  phases  would  occupy  but 
88  days,  the  time  of  Mercury's  revolution,  but 
as  the  Earth  also  moves  in  its  orbit  about  the 
Sun  in  the  same  direction  as  does  Mercury,  it 
takes  more  than  one  complete  revolution  of  the 
latter  to  bring  it  again  between  the  Sun  and 
the  Earth.  The  phases,  of  course,  result  from  the 
relative  positions  of  the  three  bodies,  the  Sun, 
Mercury,  and  the  Earth,  as  the  phases  of  the 
Moon  result  from  the  position  of  the  Moon  in 
relation  to  the  Earth  and  the  Sun. 

The  plane  of  Mercury's  orbit  is  inclined,  or 
tilted,  to  the  ecliptic  (the  plane  of  the  Earth's 
orbit)  at  an  angle  of  about  seven  degrees.  It  is 
for  this  reason  that  the  planet  does  not  pass 


22      THe  Essence  of  Astronomy 

directly  between  us  and  the  Sun  every  revolu- 
tion. About  twelve  times  a  century  it  does  pass 
directly  between  us  and  the  Sun,  and  is  then 
visible,  with  a  telescope,  as  a  small  black  dot 
upon  the  Sun's  disk.  Such  an  occurrence  is 
called  a  transit. 

Mercury  has  no  moon  of  visible  size. 

As  the  orbit  of  Mercury  is  inside  that  of  the 
Earth,  the  planet  is  never  seen  at  night,  but  is 
visible  at  certain  times  close  to  the  horizon,  only 
just  after  sunset  in  the  west,  or  just  before 
sunrise  in  the  east.  Because  of  this,  and  because 
of  the  haze  which  is  usually  prevalent  in  the 
Earth's  atmosphere,  it  is  viewed  probably  less 
than  any  other  celestial  body  visible  to  the  naked 
eye.  It  is  so  necessary  to  have  good  seeing 
conditions,  that  it  is  said  the  celebrated  astrono- 
mer Copernicus  died  without  observing  Mercury. 
This  sounds  extreme,  for  the  writer  has  picked 
it  up  several  times  with  an  opera-glass  and  naked 
eye  from  a  city  window.  When  seen  with  the 
naked  eye,  it  shines  as  a  bright  star,  showing  a 
reddish  tint  because  of  the  atmospheric  dis- 
turbances and  the  horizon  haze  through  which 
its  light  must  penetrate.  It  usually  seems  to 
"twinkle"  somewhat  for  the  same  reason, 


Mercury  23 

and,  indeed,  is  sometimes  spoken  of  as  "The 
Twinkler." 

To  avoid  these  bad  "seeing'*  conditions,  the 
astronomers  study  Mercury  in  daylight.  This 
is  possible  with  large  telescopes,  though,  of 
course,  the  planet  is  then  quite  invisible  to  the 
naked  eye,  being  lost  in  the  glare  of  the  sunlight. 

There  seems  no  possibility  of  life  as  we  know 
it  upon  Mercury.  The  atmosphere  is  of  such 
rarity  as  to  be  insufficient  to  support  such  life; 
and  the  terrific  heat  to  which  the  faithful  sun- 
ward side  of  the  planet  is  constantly  subjected 
adds  greatly  to  the  improbability.  The  lowest 
temperature  there  is  above  that  of  the  boiling 
point  of  water  here,  and  the  greatest  is  more  than 
twice  as  hot.  The  dark  side  is,  of  course,  con- 
stantly exposed  to  the  awful  cold  of  space, 
suffering  also  from  the  lack  of  an  atmospheric 
blanket.  The  planet,  therefore,  revolves  almost 
certainly  as  a  dead  world,  scorched  on  one  side, 
and  frozen  on  the  other. 

One  curious  effect  of  the  great  variation  in  the 
distance  of  Mercury  from  the  Sun  is  that  in 
about  six  weeks  (44  days)  the  latter,  viewed  from 
the  planet,  apparently  grows  to  more  than  double 
its  size  and  brilliancy,  and  in  the  next  six  weeks 


24      XHe  Essence  of  Astronomy 

shrinks  again.  The  average  size  of  the  Sun,  seen 
from  Mercury,  is  somewhat  more  than  twice  its 
size  as  viewed  from  the  Earth. 

In  spite  of  the  difficulty  experienced  in  viewing 
the  planet  with  the  unaided  eye,  we  have  records 
of  observations  of  Mercury  made  at  Nineveh 
2500  years  ago. 


CHAPTER  IV 

VENUS 

VENUS,  reckoning  on  an  ascending  scale,  is  the 
third  in  size  of  the  Sun's  family  of  planets.  It  is 
larger  than  Mercury  and  Mars,  and  just  smaller 
than  the  Earth.  It  revolves  in  an  orbit  which 
lies  between  that  of  Mercury  and  that  of  the 
Earth. 

The  diameter  is  nearly  7700  miles,  the  Earth's 
being  7918.  Its  surface  is  about  nine  tenths  that 
of  the  Earth  and  its  volume  about  nine  tenths 
also.  Venus  may  be  slightly  flattened  at  its 
poles;  no  accurate  measure  has  been  obtained 
however. 

Its  mean  distance  from  the  Sun  is  almost 
exactly  67,200,000  miles.  Its  orbit  is  the  least 
elliptical  of  all  the  planets;  so  approaching  a 
true  circle  that  the  variation  of  its  distance  from 
the  Sun  need  not  be  considered. 

Its  distance  from  the  Earth  varies  between 
25,000,000  miles  when  nearest  to  it,  to  about 
25 


26      THe  Essence  of  Astronomy 

160,000,000  miles  when  farthest  from  it  and  on 
the  other  side  of  the  Sun. 

It  revolves  about  the  Sun  in  224.7  days,  this 
being  the  year  of  Venus. 

The  rotation  upon  its  axis  is  not  definitely 
known.  It  seems  almost  certain,  however,  that, 
like  Mercury,  the  time  of  rotation  equals  the 
time  of  revolution,  and  that  the  planet  therefore 
turns  always  the  same  side  to  the  Sun.  If  this 
rotation  is  correct,  as  now  believed,  the  axis  of 
the  planet  is  perpendicular  to  the  orbit.  Recent 
observations  seem  to  substantiate  this  supposi- 
tion. Venus  therefore  lacks  seasons,  as  does 
Mercury. 

It  moves  in  its  orbit  around  the  Sun  at  an 
almost  unvarying  speed  of  22  miles  per  second. 

Its  mass  is  somewhat  less  than  eight  tenths 
that  of  the  Earth;  and  its  density  somewhat 
more  than  eight  tenths.  Venus  weighs,  there- 
fore, nearly  five  times  as  much  as  a  globe  of 
water  the  same  size. 

Its  attraction  for  bodies  on  its  surface  is  also 
somewhat  more  than  eight  tenths  that  of  the 
Earth.  A  body  weighing  200  pounds  here  would 
weigh  on  Venus  about  165  pounds.  Venus  has 
apparently  a  rather  dense  atmosphere.  The 


Venus  27 

spectroscope  shows  the  presence  of  water;  and 
one  of  the  reasons  that  definite  observations  of 
the  planet  are  so  difficult  to  obtain  is  because  of 
the  perpetual  veil  of  cloud  between  us  and  its 
surface.  ^ 

It  receives  from  the  Sun  about  twice  as  much 
light  and  heat  as  does  the  Earth.  The  cloud- veil, 
however,  probably  tempers  this  to  a  climate  not 
much  more  severe  than  our  tropics  at  times. 

Also,  because  of  the  cloud- veil  the  reflective 
power  of  the  planet  is  very  high.  It  throws  back 
over  75  per  cent,  of  the  light  it  receives  from  the 
Sun.  This  dazzling  brilliancy  adds  to  the  dif- 
ficulty of  observing. 

Viewed  with  a  telescope  Venus,  too,  shows 
''phases"  as  does  our  Moon  (Chapter  VI.). 
This  fact  was^rst  observed  by  Galileo  in  1610. 
The  period  between  "new"  to  "new"  occupies 
about  580  days. 

The  plane  of  its  orbit  is  inclined  to  that  of  the 
Earth  at  an  angle  of  about  3.5  degrees.  This 
would  seem  to  indicate  that  it  should  pass 
directly  between  us  and  the  Sun  (transit)  more 
often  than  Mercury.  However,  because  of  the 
relation  between  the  times  of  revolution  of  Ve- 
nus and  the  Earth  these  transits  are  compara- 


28      THe  Essence  of  Astronomy 

tively  rare,  occurring  about  at  the  rate  of  two  to 
the  century.  The  last  was  in  1882.  The  next 
will  be  2004. 

Venus,  like  Mercury,  has  no  moon  of  sufficient 
size  to  see. 

The  orbit  of  Venus  being,  also,  within  that  of 
the  Earth  the  planet  is  never  seen  late  at  night. 
Its  orbit,  however,  is  so  much  greater  in  diameter 
than  Mercury's  that  it  recedes ,  much  farther 
from  the  Sun,  and  when  at  the  best  distance  for 
observation  does  not  "set"  until  some  hours 
after  the  "day-star."  It  is  seen,  nevertheless, 
only  in  the  western  sky  after  sunset  or  in  the 
eastern  sky  before  sunrise. 

To  the  naked  eye  the  planet  is,  with  the  ex- 
ception of  the  Sun,  the  Moon,  and  an  occasional 
comet,  the  most  brilliant  of  all  celestial  bodies. 
It  has  twelve  times  the  brightness  of  the  most 
brilliant  star,  and  shines  with  a  silvery  light. 
When  at  its  best  it  can  be  seen  as  a  tiny  point  of 
light  in  full  daylight.  As  has  been  said,  this  very 
brilliancy  makes  it  most  difficult  to  observe,  and 
it  is,  on  the  whole,  a  disappointing  telescopic 
object.  Like  Mercury,  it  is  best  observed  in  the 
daytime,  when  some  of  its  glare  is  overcome  by 
the  daylight. 


Venxis  29 

That  there  may  be  on  the  Sunward  side  of 
Venus  a  life  approaching  that  of  our  tropics 
seems  possible.  There  are  no  known  conditions 
that  would  appear  to  prevent  the  existence  of 
such.  It  seems  hardly  probable,  however,  that 
the  atmosphere  of  the  planet  can  carry  enough 
warmth  to  its  dark  side  to  make  this  habitable. 

Venus  has  been  known  and  recognized  since 
the  earliest  days  of  astronomy. 


CHAPTER  V 

THE  EARTH 

THE  Earth  is  the  third  of  the  planets  in  order 
of  distance  from  the  Sun,  and  is  the  fourth  in 
size  reckoning  on  an  ascending  scale.  It  is  the 
largest  of  the  four  minor  or  terrestrial  planets. 

Its  mean  diameter  is  7918  miles.  It  shows 
what  on  Venus  or  Mercury  cannot  be  measured 
— a  flattening  at  the  poles,  so  that  the  diameter 
from  pole  to  pole  is  27  miles  shorter  than  the 
diameter  measured  through  the  equator.  This 
bulge  at  the  equator  was  formed,  while  the 
Earth  was  in  a  molten  or  plastic  condition,  by  the 
centrifugal  force  developed  by  its  rotation  about 
its  axis. 

Its  surface  comprises  about  187,000,000  square 
miles,  of  which  nearly  three  quarters  are  covered 
by  water.  Its  volume  is  about  260,000,000,000 
cubic  miles. 

Its  mean  distance  from  the  Sun  is  very  nearly 

93,000,000  miles.     Its  orbit  is  slightly  elliptical 
30 


The  Earth  31 

so  that  this  distance  varies  about  3,000,000 
miles.  The  Earth  is  nearest  the  Sun  early  in 
January,  and  farthest  from  it  in  early  July.  The 
mean  distance  93,000,000  miles  is  one  of  the 
astronomical  units  of  measurement. 

The  Earth  revolves  about  the  Sun  in  365  days 
6  hours  9  minutes  9  seconds.  This  is  its  true 
year.  There  are  two  other  "  years. " 

It  rotates  upon  its  axis  in  23  hours  56  minutes 
and  4  seconds. 

The  difference  between  the  various  "years" 
spoken  of,  and  the  3  minutes  and  56  seconds 
difference  between  the  time  of  rotation  and  the 
day,  cannot  be  discussed  in  the  space  available 
here.  Any  of  the  larger  general  works  listed  in 
the  bibliography  gives  a  full  explanation  of  the 
reasons,  and  the  effect  of  these  apparent  dis- 
crepancies in  time. 

The  axis  of  the  Earth  is  inclined  to  the  plane 
of  its  orbit  about  23.5  degrees.  This  inclination, 
although  actually  varying  somewhat,  may  be 
considered  as  fixed,  and  the  direction  of  the 
inclination  may  also,  for  the  moment,  be  con- 
sidered fixed;  that  is,  during  a  revolution,  the 
Earth  points  its  axis  constantly  at  the  same 
point  in  space.  It  is  this  inclination  which  is  the 


32      THe  Essence  of  Astronomy 

cause  of  our  seasons.  When  the  north  pole  of 
the  Earth's  axis  is  leaning  away  from  the  Sun, 
it  is  winter,  in  the  northern  hemisphere,  since 
the  Sun  is,  in  consequence,  above  the  horizon  for 
a  shorter  part  of  the  day ;  and  its  rays  strike  the 
surface  of  the  Earth  there  obliquely,  and  as  a 
result  with  less  power;  these  conditions  more 
than  compensating  for  the  nearer  approach  of 
the  Earth  to  the  Sun  during  the  winter  months. 
Half  a  year,  or  half  a  revolution,  later,  the  north 
pole  of  the  Earth's  axis  "is  leaning  toward  the 
Sun,  and  we  have  summer,  or  the  hot  season, 
since  conditions  are  just  the  reverse.  Of  course, 
the  seasons  in  the  southern  hemisphere  are 
exactly  the  opposite  of  those  in  the  northern, 
but  are  somewhat  more  intense,  since  it  is  sum- 
mer when  the  Earth  is  also  nearest  the  Sun,  and 
winter  when  it  is  farthest  away.  Between  win- 
ter and  summer  are  the  seasons  of  autumn  and 
spring,  when  the  axis  of  the  Earth  points  neither 
toward  nor  away  from  the  Sun  and  we  have  more 
equitable  conditions  of  temperature. 

The  Earth  moves  in  its  orbit  with  a  mean 
speed  of  nearly  18.5  miles  a  second,  covering 
in  its  revolution  each  day  about  1,500,000 
miles.  It  moves  slightly  faster  in  January 


THe  EartH  33 

when  nearest  the  Sun,  and  slower  in  July  when 
farthest  away. 

Its  density  is  a  little  more  than  5.5  times  that 
of  water.  A  ball  the  same  size  made  entirely  of 
granite  would  weigh  only  about  one  half  as  much 
as  the  Earth. 

Its  atmosphere  is  comparatively  dense,  hav- 
ing at  sea  level  a  pressure  of  about  15  pounds  to 
the  square  inch.  This  atmosphere  acts  as  does 
the  glass  of  a  hot-house,  preventing  the  Earth 
on  its  night  side  from  radiating  all  its  day- 
acquired  heat  into  space.  The  atmosphere  also 
tempers  the  heat  of  the  Sun's  rays  as  they  strike 
the  Earth. 

Were  it  not  for  the  air  and  dust  suspended  in 
the  air  the  diffusion  of  daylight  would  not  occur. 
Where  the  Sun's  rays  fell  direct^ there  would  be 
brilliant  illumination,  but  elsewhere  the  shadows 
would  be  inky  black.  The  light  is  now  spread  by 
the  progressive  reflection  from  the  countless 
ever-present  dust  particles.  It  is  only  this  re- 
fraction and  diffusion  of  the  Sun's  light  that 
prevents  our  seeing  the  stars  in  the  daytime. 
Without  it  the  sky  would  be  a  dead  black,  set 
with  the  brilliance  of  the  stars  as  steady  un- 
t winkling  points,  and  the  Sun  would  appear  as 


34      THe  Essence  of  Astronomy 

a  glowing  white  disk.  From  an  observational 
astronomer's  point  of  view  nothing  could  be 
better. 

The  atmosphere  also  protects  us  from  the 
constant  bombardment  of  meteors  (see  Chapter 
XV.),  which  are  fused  to  dust  before  they  reach 
the  Earth  itself.  It  is  partly  from  watching  and 
studying  the  flights  of  meteors  through  the  air 
that  the  latter  has  been  estimated  to  extend 
about  100  miles  from  the  Earth's  surface.  It 
is  too  rare  for  us  to  breathe  at  a  height  of  about 
seven  miles. 

Owing  also  to  the  rather  dense  atmosphere  it 
is  probable  that  the  reflecting  power  (albedo) 
of  the  Earth  is  high. 

It  is  the  plane  of  the  Earth's  orbit  (the 
ecliptic)  which  astronomers  take  as  the  stand- 
ard plane  of  the  planetary  revolutions,  com- 
paring the  inclinations  of  the  other  planetary 
orbits  with  it. 

The  Earth  is  still  at  a  high  temperature. 
Owing  to  the  tremendous  pressure  it  is  impossible 
that  it  be  now  a  molten  ball  with  a  crust  sur- 
rounding it,  as  was  believed  for  many  years. 
The  heat  at  a  great  depth,  however,  is  probably 
quite  hot  enough  to  melt  the  most  refractory 


The  EartH  35 

substances,  were  they  not  compressed  to  such 
an  extent.  The  study  of  earthquake  shocks  has 
demonstrated  that,  as  a  whole,  the  Earth  is 
nearly  as  rigid  as  a  steel  ball. 

The  motions  of  the  Earth  in  space  are  very 
complex.  The  more  evident  of  these  are  the 
yearly  revolution,  and  the  daily  rotation.  Be- 
side these,  there  are,  the  great  motion  together 
with  the  Sun  and  the  rest  of  the  planets  in  the 
general  direction  of  the  brilliant  star  Vega,  at 
the  rate  of  about  12  miles  a  second;  and  eight 
other  motions,  some  of  them  "wobbles"  on  its 
axis  like  an  unevenly  spinning  top,  and  others 
waverings  in  its  orbit — because  of  the  changing 
relative  direction  of  the  Moon  and  the  other 
planets,  and  the  consequent  variation  of  gravita- 
tional pull.  There  are  in  all  eleven  motions. 

One  of  the  small  "wobbles"  of  the  Earth 
produces  an  actual  wandering  of  the  poles,  and 
a  consequent  ceaseless  change  in  the  latitude  of 
every  place  upon  the  globe.  This  wandering  is  a 
matter  of  but  a  few  yards,  yet  it  can  be  followed 
with  great  accuracy,  since  the  change  in  latitude 
causes  a  measurable  shift  in  the  position  of  the 
stars. 

It  is  interesting  to  know  that,  though  the 


36      TKe  Essence  of  Astronomy- 
Earth's  orbit  in  relation  to  the  Sun  is  a  closed 
curve,  this  deviates  from  a  straight  line  only 
about  one  ninth  of  an  inch  in  18.5  miles,  the 
distance  traversed  by  the  Earth  in  one  second. 

The  Earth  possesses  a  magnetic  force  which 
directs  the  needle  of  the  compass.  This  force  is 
subject  to  periodic  and  sporadic  changes  which 
cannot  be  discussed  here.  In  passing,  however, 
it  may  be  said  that  many  of  these  changes, 
possibly  all  of  them,  are  produced  by  disturb- 
ances in  the  Sun;  the  regular  "variation  of  the 
magnetic  needle"  being  synchronous  with  the 
Sun-spot  cycle  of  about  eleven  years. 

It  is  this  magnetic  force  that  is  in  some  way 
responsible  for  the  Aurora,  that  marvelous  lu- 
minous display  which  is  witnessed  often  in  high 
latitudes  both  northern  and  southern,  the  exact 
explanation  for  which  has  not  yet  been  found. 

The  magnetic  poles  of  the  Earth  are  not 
located  at  the  poles  of  its  axis  of  rotation,  but 
at  considerable  distances  from  them.  Neither 
are  these  magnetic  poles  constant  in  position, 
their  periodic  and  regular  change  of  location 
giving  rise  to  the  variation  of  the  compass 
just  spoken  of. 

One  of  the  effects  of  the  comparatively  rapid 


THe  EartH  37 

rotation  of  the  Earth  is  that,  at  the  equator, 
everything  weighs  less  (that  is,  is  pulled  toward 
the  Earth's  center  with  less  force)  than  at  any 
point  farther  north  or  south.  At  the  equator 
the  centrifugal  force  developed  by  the  spin  of 
the  Earth  is  at  its  strongest,  and  lessens  in 
power  as  the  latitude  increases  until,  at  the 
poles,  it  ceases  to  exist.  A  spot  at  the  equator 
travels  at  over  a  thousand  miles  an  hour,  while 
at  the  poles  it  is  merely  turned  around  once  in 
twenty-four  hours.  Also,  the  fact  that  the  polar 
diameter  of  the  Earth  is  less  than  the  equatorial 
diameter  places  a  point  at  the  pole  somewhat 
nearer  the  center  of  the  Earth  than  a  point  on 
the  equator.  This  aids  in  lending  an  object 
weight. 

This  difference  in  weight  is  slight,  but  is 
readily  measurable,  of  course  only  with  a  spring 
scale.  In  a  weight  balance  the  marked  weights 
are  affected  in  exactly  the  same  proportion  as 
the  objects  weighed,  and,  therefore,  no  difference 
would  be  shown. 


CHAPTER  VI 

THE  MOON 

THE  Moon  is  the  only  visible  satellite  of  the 
Earth,  and  revolves  about  it.  It  is  probable 
that  all  the  planets,  and  their  larger  satellites, 
including  the  Earth  and  the  Moon,  swing  in 
orbits  about  them  stray  bits  of  "world-dust" 
that  their  attraction  has  captured,  but  as  re- 
gards the  Moon  and  the  near  planets  these  must 
be  very  small  not  to  have  been  discovered. 

The  diameter  of  the  Moon  is  2162  miles;  like 
Venus  and  Mercury  it  shows  no  polar  flattening. 

Its  surface  contains  about  15,000,000  square 
miles;  about  one  thirteenth  that  of  the  Earth. 
Its  volume  is  about  one  fiftieth  that  of  the 
Earth.  The  dimensions  of  the  Moon  are  much 
greater  in  proportion  to  the  Earth  than  those  of 
any  other  satellite  to  its  primary. 

Its  mean  distance  from  the  Earth  is  nearly 
239,000  miles.  Its  orbit,  like  all  celestial  orbits, 
is  more  or  less  elliptical  and  its  distance  may 
38 


TKe  Moon  39 

vary  between  slightly  more  than  221,000  miles 
and  253,000  miles.  Its  usual  variation  is  such, 
however,  that  in  one  revolution  it  amounts  to  a 
difference  in  distance  from  us  of  about  25,000 
miles. 

It  revolves  around  the  Earth  in  27  days  7 
hours  43  minutes  and  11.5  seconds. 

It  rotates  upon  its  axis  in  the  same  time,  and, 
in  consequence,  always  keeps  the  same  side 
turned  to  the  Earth,  as  do  Mercury  and  Venus 
to  the  Sun.  It  is  probable  that  nearly  all  the 
satellites  of  the  planets  have  this  same  relation 
between  their  revolution  and  their  rotation. 
Because  of  certain  variations  in  the  Moon's 
motion,  both  a  "nodding"  and  a  "shaking"  of 
its  head,  so  to  speak,  we  see  first  somewhat 
around  one  side  and  then  somewhat  around  the 
other;  in  all  about  four  sevenths  of  the  entire 
surface  can  be  seen.  Its  axis  is  about  perpen- 
dicular to  the  plane  of  its  orbit. 

The  Moon  moves  in  its  revolution  around  the 
Earth  at  a  mean  speed  of  about  37  miles  per 
minute. 

Its  mass  is  about  s\  that  of  the  Earth;  its 
density  about  three  fifths,  or  nearly  three  and 
one  half  times  that  of  water.  Its  attraction  for 


40      TKe  Essence  of  Astronomy- 
bodies  at  its  surface  is  about  one  sixth  that  of 
the  Earth.     A  body  weighing  200  pounds  here 
would  weigh  about  34  pounds  on  the  Moon. 

It  has  no  atmosphere,  or  at  least,  if  any,  but 
the  veriest  ghost  of  one.  It  is  for  this  reason  that 
the  details  of  the  lunar  surface  are  so  clear-cut 
when  viewed  through  a  telescope,  and  that  the 
shadows  are  so  intensely  black.  Except  where 
the  sunlight  strikes  direct — there  is  no  light.  To 
step  into  a  shadow  on  the  Moon  would  be  to 
disappear. 

The  lack  of  an  atmosphere  means  that  there 
is  no  lunar  "weather. "  There  is  never  any  rain ; 
in  fact  there  is  no  liquid  water  on  the  Moon. 
There  is  no  wind.  There  are  no  unexpected 
changes  in  temperature. 

The  amount  of  light  and  heat  received  from 
the  Sun  is  about  the  same  as  intercepted  by  the 
Earth.  The  Moon,  however,  having  no  atmos- 
phere, radiates  away  its  heat  almost  as  soon  as 
it  is  received.  It  seems  probable  that  the  surface 
is  very  hot  during  the  lunar  day,  and  it  is 
inevitable  that,  during  the  lunar  night,  the  sur- 
face temperature  drops  appallingly  low — almost 
to  the  absolute  cold  of  space. 

The  reflective  power  of  the  Moon's  surface  is 


TKe  Moon  41 

not  high,  in  spite  of  its  apparent  brilliance  when 
full.  It  throws  back  only  about  17  percent,  of 
the  light  striking  it.  If  the  sky  were  covered 
with  full  moons  we  should  receive  from  them 
only  about  one  eighth  part  of  the  light  we  get 
from  the  Sun. 

The  Moon  reflects  to  us  a  certain  amount  of 
heat;  so  small  is  this,  however,  that  it  requires 
the  most  delicate  instruments  even  to  demon- 
strate this  fact.  The  amount  has  never  been 
accurately  measured. 

The  phases  of  the  Moon  were  probably  among 
the  first  celestial  phenomena  to  be  noticed  by 
man.  Since  the  Moon  is  an  opaque  body,  and 
shines  by  reflected  light  only,  we  can  see  but  that 
part  which  is  illuminated  and  at  the  same  time 
turned  toward  us.  When  the  Moon  is  between  us 
and  the  Sun  (see  diagram)  its  dark  side  is  turned 
toward  us  and  is  entirely  invisible.  This  is  at 
the  time  of  the  "new  moon."  About  a  week 
later  half  of  the  illuminated  hemisphere  is  visible. 
This  is  "first  quarter."  "  Last  quarter"  occurs 
about  one  week  after  full  moon,  the  other  half  of 
the  Earthward  hemisphere  being  illuminated 
then.  When  the  Moon  is  on  the  other  side  of 
the  Earth  from  the  Sun  it  presents  to  us  the  full 


LAST  OUARTCP 


I  NEW  MOON 


FULL    MOON\ 


/»r  QUARTER 


THE    PHASES   OF   THE    MOON 

A  5  */»<?  Moon  swings  around  the  Earth  as  shown  by  the  small  arrows,  the 

Sun  being  in  the  direction  indicated  by  the  long  arrows,  it  is  evident 

that,  at  "  new  moon,"  the  dark  side  of  the  Moon  will  be  turned  to 

the  Earth,  and  as  the  satellite's  revolution  continues  an 

increasing  portion  of  the  illuminated  hemisphere  will 

be  seen,  until  at  "full  moon  "  the  whole  of  it  is 

presented  to  us.     From  "full  "  to  "  new  "  a 

corresponding  decrease  occurs 


The  Moon  43 

illuminated  hemisphere.  Between  the  periods  of 
new  moon  and  first  quarter  (or  half  moon,  as 
it  is  popularly  called)  we  see  illuminated  less 
than  half  of  a  hemisphere  and  we  have  then  a 
"crescent"  moon.  Between  the  half  moon  and 
the  full  we  see  more  than  half  the  illuminated 
hemisphere  and  have  the  "gibbous"  moon.  The 
line  between  the  illuminated  and  dark  hemi- 
spheres is  called  the  "terminator." 

Mercury  and  Venus  have  just  such  phases, 
for  the  same  reason.  There  is  this  difference 
however.  When  the  Moon  is  at  "full"  we  are 
between  it  and  the  Sun ;  when  Mercury  or  Venus 
show  "full"  the  Sun  is  between  us  and  them. 
Viewed  from  the  Moon,  the  Earth  would  show 
phases  also,  being  "full  Earth"  at  the  time 
of  new  moon,  and  "new  Earth"  at  the  time  of 
full  moon. 

If  the  orbit  of  the  Moon  around  the  Earth 
were  in  the  same  plane  as  that  of  the  Earth's 
revolution  about  the  Sun  we  should  have  eclipses 
of  the  Sun  every  new  moon  and  eclipses  of  the 
Moon  every  full  moon.  (For  eclipses  see  Chapter 
XV.)  The  lunar  orbit,  however,  is  inclined  to 
that  of  the  Earth  at  a  little  more  than  5  degrees. 
Therefore  at  "new"  the  Moon  usually  passes  a 


44      THe  Essence  of  Astronomy- 
little  below  or  above  the  Sun,  and  at  "full" 
above  or  below  the  shadow  of  the  Earth. 

There  is  a  popular  superstition  that  the  Moon 
affects  the  weather,  but  the  most  careful  study 
of  records  has  failed  to  show  any  such  connection. 
It  is  also  believed  that  the  full  moon  clears 
away  clouds,  but  again  scientific  research  does 
not  disclose  any  such  effect.  Whether  the  dif- 
ference of  the  Moon's  distance  from  the  Earth, 
caused  by  the  former's  elliptical  orbit,  actually 
affects  the  magnetic  needle,  as  indeed  it  seems 
to  do,  is  a  question  now  being  closely  studied. 

Near  the  time  of  new  moon,  the  whole  disk  is 
easily  seen,  the  part  upon  which  the  Sun  does 
not  shine  being  illuminated  by  a  pale  somewhat 
reddish  light.  This  is  popularly  known  as  the 
"old  moon  in  the  new  moon's  arms."  This 
light  is  reflected  light  from  the  Earth,  or  Earth- 
shine.  As  noted  above,  the  Earth  at  that  time, 
when  viewed  from  the  Moon,  is  nearly  full. 
The  ruddy  color  of  the  Earth-shine  is  caused  by 
the  light  passing  twice  through  our  atmosphere. 
Leonardo  da  Vinci  was  the  first  man  to  explain 
this  phenomenon  satisfactorily. 

The  surface  of  the  Moon  is  broken  into  great 
mountain  ranges,  enormous  "craters,"  wide  and 


THe  Moon  45 

deep  cracks  or  "clefts, "  and  innumerable  smaller 
cracks  or  rills.  The  whole  of  its  visible  surface 
has  been  mapped  and  measured  more  carefully 
and  accurately  than  that  of  the  Earth,  and  over 
30,000  craters  have  been  counted  on  the  Earth- 
ward hemisphere.  The  first  map  of  the  Moon 
was  made  in  1647.  The  height  of  the  mountains 
is  enormous  in  proportion  to  the  size  of  the 
Moon,  several  being  over  18,000  feet  high.  The 
craters  vary  in  size  from  tiny  pits  to  vast 
cavities  over  100  miles  in  diameter.  Many  of 
the  larger  craters  have  great  central  mountain 
peaks  rising  from  their  floors,  and  many  others 
show  smaller  craters  within  their  boundaries. 
The  lack  of  "weather"  allows  these  crater  walls 
and  mountain  peaks  to  remain  most  precipitous. 
On  the  Earth  the  "weather"  would  and  does 
wear  such  to  more  gentle  slopes. 

The  causes  of  this  condition  of  the  Moon's 
surface  are  not  definitely  known.  It  seems 
probable  that  they  were  volcanic  forces  which 
were  able  to  accomplish  so  much  more  than  on 
th§  -.Earth  because  of  the  much  less  weight  of 
matter  upon  the  Moon.  There  is  some  support 
to  the  theory  that  meteoric  bombardment  is 
partially  responsible. 


46     THe  Essence  of  Astronomy 

From  some  of  the  bright  crater-mountains 
emanate,  or  rather  apparently  emanate,  systems 
of  bright  ray  streaks,  extending  in  some  cases 
nearly  across  a  visible  hemisphere  of  the  Moon. 
These  are  seen  only  under  high  illumination,  and 
show  best  at  full  moon.  In  the  accompanying 
illustration,  the  greatest  of  these  ray-systems, 
that  with  the  crater  Tycho  as  its  center,  is  just 
beginning  to  show.  It  is  these  ray  streaks  of  the 
Tycho  system  which  give  to  the  full  moon  an 
appearance  somewhat  like  that  of  a  peeled 
orange.  No  plausible  and  really  satisfactory 
explanation  of  these  ray  systems  has  yet  been 
put  forward. 

To  Earth-dwellers  the  greatest  value  of  the 
Moon  is  its  influence  upon  the  tides,  which  by 
their  constant  change  flush  our  harbors  and 
shores,  and  by  their  motion  prevent  a  stagnation 
of  the  ocean  certain  to  occur  without  them. 

Observations  of  recent  years  are  causing  as- 
tronomers to  waver  from  the  opinion  cur- 
rent during  the  past  century,  namely  that  the 
Moon  is  a  dead,  unchanging  mass.  Some  well- 
known  observers  have  gone  so  far  as  to  state 
flatly  that  there  are  evident  signs  of  a  vegetation 
existing  on  the  floors  of  some  of  the  larger  craters. 


THE    MOON,    AT    NINE    AND   THREE-QUARTER    DAYS 

Image  inverted,  as  in  astronomical  telescope 

.Note  the  faint  streaks  radiating  from  Tycho,  the  almost  circular  crater  in  the 
upper  part  of  the  picture.     The  dark  crater  near  the  bottom  of  the  photo- 
graph is  Plato,  where  color-changes  have  been  observed  on  the 

moon's  surface 
From  a  photograph  taken  at  the  Yerkes  Observatory 


THe  Moon  47 

It  is  certain  that  slight  changes  in  shade  do 
occur  in  some  regions  of  the  lunar  surface,  but 
the  necessarily  extreme  rarity  of  what  atmos- 
phere the  Moon  may  still  retain  seems,  in  the 
opinions  of  even  the  non-conservative  astrono- 
mers, to  place  out  of  the  question  the  existence 
of  actual  vegetation,  even  in  the  form  of  the  most 
primitive  mould. 

That  the  Moon  is  not  wholly  dead,  as  far  as 
surface  change  is  concerned,  seems  almost  prob- 
able. Certain  small  craters  appear  to  show 
unstable  shapes,  as  if  there  still  stirred  a  faint 
volcanic  activity. 


CHAPTER  VII 

MARS 

MARS  is  fourth  planet  in  distance  from  the 
Sun,  and  next  to  the  smallest  in  size,  being  larger 
only  than  Mercury. 

Its  mean  diameter  is  4230  miles,  a  little  more 
than  half  that  of  the  Earth.  Its  polar  flattening 
is  small.  Its  surface  is  about  one  quarter  that 
of  the  Earth,  and  its  volume  about  one  seventh. 

Its  mean  distance  from  the  Sun  is  nearly 
141,500,000  miles.  Its  orbit  is  the  most  eccen- 
tric of  all  the  planets  except  Mercury,  and  its 
distance  from  the  Sun  varies  between  about 
154,500,000  miles  and  128,200,000  miles. 

Its  distance  from  the  Earth  varies  between 
very  nearly  35,000,000  miles  and  248,000,000 
miles. 

The  relation  of  the  times  of  the  revolutions  of 
Mars  and  the  Earth  about  the  Sun  are  such  that 
the  Earth  passes  between  Mars  and  the  Sun  only 

once  every  two  years.    It  is  only  when  the  Earth 
48 


Mars  49 

comes  between  the  Sun  and  Mars,  when  the 
latter  is  at  its  nearest  point  to  the  Sun,  that  the 
35,000,000  miles  minimum  distance  is  reached. 
This  is  about  every  15  years.  When  the  Earth 
passes  between  the  two  when  Mars  is  farthest 
from  the  Sun,  the  planet  is  about  61,000,000 
miles  away  from  us.  It  is  at  the  times  of  these 
near  approaches  of  Mars  that  the  public  interest 
in  the  planet  is  re-awakened  by  the  heralded 
preparations  in  the  observatories  to  take  advan- 
tage of  this  best  opportunity  for  observation. 

Mars  revolves  about  the  Sun  in  very  nearly 
687  days.  This  is  the  year  of  Mars. 

It  rotates  upon  its  axis  in  24  hours  37  minutes 
and  22  seconds,  giving  it  a  day  slightly  longer 
than  that  of  the  Earth. 

Its  axis  is  inclined  to  the  plane  of  its  orbit 
nearly  25  degrees,  somewhat  more  than  that  of 
the  Earth.  This  means  a  very  little  greater  range 
in  the  seasonal  temperatures  than  the  Earth 
undergoes.  The  length  of  the  Martian  year 
produces,  of  course,  seasons  longer  proportion- 
ately than  those  of  our  planet. 

Mars  moves  in  its  orbit  around  the  Sun  at  a 
mean  speed  of  15  miles  a  second. 

Its  mass  is  just  one  ninth  that  of  the  Earth, 


5°      THe  Essence  of  Astronomy 

and  its  density  about  three  quarters.  Its  attrac- 
tion for  bodies  at  its  surface  is  nearly  two  fifths 
that  of  the  Earth;  a  body  weighing  200  pounds 
would  weigh  but  80  on  Mars. 

Mars  has  an  atmosphere  about  which  there 
is  much  discussion.  That  it  is  less  dense  than 
ours  is  universally  admitted.  The  question  of 
whether  there  is  water-vapor  in  this  Martian 
air,  and  consequently  liquid  water  on  its  surface, 
is  the  great  point  of  dispute.  It  would  seem  that 
recent  spectroscopic  observations  prove  the  af- 
firmative side  of  the  debate. 

Mars  receives  from  the  Sun,  on  an  average, 
less  than  half  of  the  light  and  heat  which  strikes 
the  Earth;  when  nearest  the  Sun  about  52  per 
cent,  and  when  farthest  away  about  36  per 
cent.  It  is  also  a  question  of  dispute  as  to 
whether  the  mean  temperature  is  sufficient  to 
allow  water  to  remain  liquid.  The  apparent 
melting  of  the  polar  caps,  spoken  of  later,  would 
seem  to  prove  that  water  can,  and  does,  exist  in 
a  liquid  condition. 

Its  reflective  power  is  about  twice  that  of 
Mercury  and  half  again  as  great  as  that  of  the 
Moon.  It  casts  back  about  26  per  cent,  of  the 
light  falling  upon  it. 


Mars  51 

It  shows  no  complete  phases  as  do  the  interior 
planets,  but,  when  at  right  angles  with  us 
and  the  Sun,  it  presents  a  gibbous  phase  like 
that  of  the  Moon  a  few  days  before  or  after 
"full." 

The  orbit  of  Mars  is  inclined  to  that  of  the 
Earth  at  slightly  less  than  two  degrees.  Viewed 
from  Mars,  the  Earth  "transits"  the  Sun,  at 
intervals  of  from  75  to  100  years,  as  do  Venus 
and  Mercury  seen  from  here. 

Mars  has  two  moons,  both  very  small.  Their 
sizes  are  not  accurately  known.  The  most 
powerful  telescope  cannot  magnify  them  enough 
to  allow  ordinary  measurement.  Their  sizes 
are  estimated,  from  the  amount  of  light  they 
reflect,  at  from  5  to  30  miles  in  diameter.  The 
outer  one,  Deimos,  revolves  in  30  hours  18 
minutes,  at  a  distance  from  the  center  of  Mars  of 
14,700  miles,  and  the  inner  one,  Phobos,  in  7 
hours  39  minutes,  at  a  distance  of  5900  miles 
from  the  center  and  only  about  3800  miles  from 
the  surface.  As  Mars  rotates  in  about  24^ 
hours  Deimos  is  seen  by  the  Martians,  if  there 
are  such,  to  rise  very  slowly  in  the  east  and  to 
pass  with  great  leisure  across  the  sky  remaining 


52     XKe  Essence  of  Astronomy 

above  the  horizon  for  about  two  days  and  a 
half  at  a  time,  and  passing  through  its  phases 
nearly  twice  in  this  period. 

Phobos  on  the  other  hand,  rises  in  the  west, 
hurries  across  the  sky,  sets  in  the  east,  and  rises 
again  in  the  west  about  four  hours  later.  Three 
times  in  the  course  of  a  Martian  day  does 
Phobos  rise  and  set,  also  passing  through  its 
phases  each  crossing  of  the  sky.  It  has  been 
remarked  that,  to  Martians,  Phobos  might  serve 
as  a  fairly  accurate  time-piece.  Its  orbital  speed 
is  about  40  miles  per  minute. 

The  smaller  satellite  is  unique  as  being  the 
only  one  known  which  has  a  period  of  revolution 
less  than  the  rotational  period  of  its  primary. 

One  of  the  results  of  the  nearness  of  the 
satellites  is  that,  from  the  surface  of  Mars,  the 
inner  one,  Phobos,  is  not  visible  from  above 
latitude  69°  in  either  hemisphere.  Mars  being 
much  smaller  than  the  Earth,  its  surface  curva- 
ture is  greater,  and  the  horizon  of  a  Martian 
place,  in  consequence,  smaller.  It  is  readily 
shown  that  above  the  latitude  mentioned,  Pho- 
bos would  never  rise  above  the  horizon,  but 
circle  constantly  below  the  bulge  of  the  planet. 
Deimos,  the  outer  satellite,  may  be  seen  from 


Mars  53 

nearly  all  points  of  Mars,  not  being  hidden  until 
latitude  82°  is  reached.  Our  own  Moon  is  at 
such  a  distance  from  us,  that  it  is,  of  course, 
visible  from  even  our  Poles. 

Frequent  eclipses  of  the  satellites  also  result 
from  their  propinquity  to  Mars;  in  some  cases 
the  moon  passing  into  the  shadow  of  the  planet 
almost  as  soon  as  it  has  risen,  and  in  many  in- 
stances rising  eclipsed.  It  has  been  calculated 
that  the  Sun  may  be  eclipsed  by  Phobos  about 
1400  times  a  year.  Naturally,  because  of  the 
speed  with  which  both  the  moons  revolve,  the 
duration  of  their  eclipses  is  shorter  than  our 
eclipses. 

The  occultation,  or  eclipse,  of  Deimos  by 
Phobos  must  occur  about  every  ten  hours  when 
viewed  from  or  near  the  Martian  equator. 

A  still  further  peculiarity  is  that  the  moons, 
when  in  the  zenith,  must  appear  much  larger 
than  when  on  the  horizon.  Since  the  diameter 
of  Mars  is  4230  miles,  Phobos,  for  example,  is 
nearly  one  half  again  (2115  miles,  or  half  the 
diameter)  as  far  away  when  rising  as  it  is  when 
overhead,  and  consequently  must,  in  mounting 
to  the  zenith,  appear  to  grow  nearly  twice  in 
area. 


54  THe  Essence  of  Astronomy- 
It  is  interesting  to  note  that  both  Voltaire,  in 
Micromegas,  and  Swift,  in  Gulliver's  Travels,  de- 
scribed Mars  with  two  small  moons ;  Swift  even 
closely  approximated  their  periods,  much  to  the 
amusement  of  astronomers  for  years,  until  the 
discovery  of  the  real  satellites  left  the  scientific 
world  amazed  at  the  accuracy  of  these  wild 
guesses. 

The  surface  of  Mars,  on  the  whole,  has  a 
characteristic  reddish-ochre  tint,  and  it  is  often 
referred  to  as  the  red,  or  ruddy,  planet.  Parts 
of  its  surface  seem  subject  to  change  in  color 
during  its  seasons,  and  that  there  is  a  real  vegeta- 
tion there  is  little  doubted.  Its  poles  are  covered 
with  white  caps  generally  believed  to  be  snow. 
It  is  from  these  caps  that  the  ' '  canals ' '  seem  to 
extend  during  the  Martian  summers  when  the 
caps  decrease  as  if  melting.  The  "canals"  are 
very  difficult  to  see,  and  appear  only  as  extremely 
fine  "hair-lines."  Many  observers  still  doubt 
their  existence  and  believe  them  to  be  illusions 
of  distance,  others  however,  show  maps  of  a 
complete  network  of  them  surrounding  the  whole 
planet.  Through  the  courtesy  of  Professor 
Lowell,  the  writer  has  in  his  possession  a  print 


ft     1 1 

s  ii?ii! 

I  i^;ii 


at  "2 
t 


•s  ° 


o    g 

ft,  R< 


iu  3  §  ^ 


Mars  55 

of  an  actual  photograph  of  Mars  made  by  Mr. 
E.  C.  Slipher  in  1907.  Since  this  photograph 
clearly  shows  some  of  the  "  canals,"  there  can 
be  no  question  that  these  lines  do  exist.  It 
is  most  unfortunate  that  it  is  impossible  to 
print  a  reproduction  of  this  photograph.  The 
detail  is  so  delicate  that  no  process  except  a 
photographic  print  made  from  a  negative  can 
show  it  satisfactorily.  The  upper  two  drawings 
in  the  accompanying  illustration  were  made  by 
Professor  Lowell  for  close  comparison  with  the 
photograph,  which  indeed  shows  more  detail  than 
the  drawings.  Professor  Lowell  insists  that  what 
is  seen  is  the  vegetation  flanking  the  piping  of  a 
vast  irrigation  system  which  taps  the  melting 
polar  snow-caps,  the  whole  planned  and  managed 
by  intelligent  beings  to  make  the  best  use  of  a 
weak  and  failing  water  supply. 

The  question  of  the  possibility  of  life  on  Mars 
rests  mainly  on  two  things ;  the  temperature  and 
water.  If  the  temperature  is  too  low  to  allow 
water  to  remain  liquid,  life,  as  we  know  it,  can- 
not exist.  The  temperature  is  by  all  estimates 
close  to  this  critical  point.  If  there  is  no  water 
at  all  or  practically  none — as  some  astronomers 
believe — life  could  not  exist.  It  would  seem 


56      XKe  Essence  of  Astronomy 

now  that,  except  for  the  ultra-conservative  as- 
tronomers, the  authorities  agree  that  water 
does  exist  in  a  free  and  liquid  state  during  the 
proper  seasons,  so  that  at  least  a  primitive 
form  of  life  can  be  supported.  It  is  not  im- 
probable that  by  the  time  of  the  next  nearest 
approach  of  Mars,  in  1924,  improved  observa- 
tional aids  will  settle  the  disputed  questions. 

Much  has  been  written  upon  this  great  Martian 
question.  Professor  Lowell's  books  are  the  most 
noted.  A  small  volume  by  Maunder  (Are  the 
Planets  Inhabited?)  gives  an  interesting  pre- 
sentation of  negative  opinions. 


CHAPTER  VIII 

THE  ASTEROIDS 

THE  Asteroids  are  a  multitude  of  very  small 
planets  revolving  about  the  Sun  between  the 
orbits  of  Mars  and  Jupiter. 

Their  diameters  range  from  probably  a  few 
yards  up  to  about  500  miles.  Their  surfaces 
and  volumes  of  course  range  accordingly. 

Their  mean  distances  differ  very  much, — from 
about  135,500,000  miles  to  over  400,000,000 
miles. 

Their  distances  from  the  Earth  vary  from 
13,500,000  miles  to  500,000,000  miles. 

Their  periods  of  revolution  range  from  687 
days  to  about  9  years. 

Of  their  rotations  nothing  at  all  definite  is 
known. 

The  average  inclination  of  their  orbits  to  the 

plane  of  the  Earth  is  nearly  8  degrees.     The 

orbit  of  one  is  inclined  nearly  35  degrees.    Their 

orbits  are  all  very  eccentric.     Their  speeds  in 

57 


58     THe  Essence  of  Astronomy 

their  orbits  vary  with  their  distances  from  the 
Sun. 

Of  the  individual  masses  and  densities  of  the 
majority  nothing  is  known.  From  their  tiny 
size  it  is  certain,  however,  that  their  attraction 
for  bodies  on  their  surfaces  must  be  very  slight. 
A  man  on  the  largest  one  could  probably  with 
ease  throw  a  stone  with  a  speed  sufficient  to 
carry  it  away  forever  from  the  little  planet. 
It  is  estimated  that  their  aggregate  mass  cannot 
exceed  J4  that  of  the  Earth.  It  may  be  as  low 
as  y£o  that  of  the  Earth. 

They   can   certainly   have   no   atmospheres. 

The  Asteroids  are  numbered  in  the  order  of 
their  discovery,  and  all  but  those  lately  found 
bear  names  also,  such  as:  Ceres  (i),  Eros  (433), 
Pallas  (2).  Ceres,  the  first  and  largest,  diameter 
about  485  miles,  was  discovered  in  1801,  and  the 
succeeding  three  within  the  next  few  years. 
Then  none  were  found  until  1845.  From  that 
date,  their  known  number  has  increased  rapidly, 
photography  lending  most  efficient  aid,  until 
now  over  800  are  listed;  and  there  are  unques- 
tionably many  more. 

They  are  probably  of  irregular  shape  and 
reflective  power,  the  variations  in  the  light  of 


TKe  Asteroids  59 

some  of  them  making  this  deduction  sound. 

Vesta  (4),  though  third  in  size,  is  the  only 
one  seen  by  the  naked  eye,  as  a  barely  visible 
speck  of  light;  and  this  only  under  favorable 
circumstances. 

One  of  these  little  planets,  Eros  (433),  is  of 
particular  interest.  This ' '  worldlet ' '  approaches 
the  Earth  at  times  nearer  than  any  other  celes- 
tial body  except  the  Moon,  meteors,  and  possible 
comets;  its  least  distance  is  13,500,000  miles. 
These  near  approaches  unfortunately  are  very 
rare,  because  of  the  relations  of  its  orbit  and 
revolution  to  those  of  the  Earth.  The  next 
one  will  not  occur  until  1938. 

The  value  of  the  proximity  of  Eros  lies  in  the 
opportunity  given  to  use  it  in  determining  with 
still  greater  accuracy  the  distance  of  the  Earth 
from  the  Sun. 

Eros  is  the  nearest  of  the  Asteroids  to  the  Sun, 
and  revolves  about  it  in  687  days,  nearly  the 
same  as  Mars,  at  a  mean  distance  of  135,480,000 
miles.  Its  orbit  like  that  of  the  others  is  very 
eccentric,  and  its  distance  from  the  Sun  varies 
between  166,000,000  miles  when  farthest  away 
and  105,200,000  miles  when  nearest. 

It  is  too  small  to  be  measured  with  any  degree 


60      XHe  Essence  of  Astronomy 

of  accuracy,  but  its  diameter  is  believed  to  be 
about  20  miles. 

There  is  a  periodic  variation  in  its  light  which 
leads  to  the  probably  sound  deduction  that  it 
rotates  on  its  axis  in  about  5^2  hours. 

Its  orbit  is  inclined  to  that  of  the  Earth  at 
nearly  eleven  degrees. 

The  largest  four  of  the  Asteroids  are,  in  the 
order  of  their  discovery: 

Ceres  (i)  Diameter  about  480  miles 
Pallas  (2)  "      300      " 

Juno    (3)  "  "      120      " 

Vesta  (4)  (The  brightest)  "  "      240      " 

It  is  improbable  that  any  of  the  others,  either 
those  now  known  or  those  yet  undiscovered, 
can  be  more  than  50  to  60  miles  in  diameter. 


CHAPTER  IX 

JUPITER 

JUPITER,  the  giant  of  the  planets,  is  the  fifth  in 
order  from  the  Sun. 

Its  mean  diameter  is  86,500  miles,  or  over  ten 
times  that  of  the  Earth.  Its  polar  flattening  is 
great,  and  the  disk  in  a  telescope  shows  distinctly 
oval.  The  equatorial  diameter  is  about  88,400 
miles,  and  the  polar  diameter  only  about  83,000. 
Its  surface  is  the  vast  amount  of  22,480,000,000 
square  miles,  and  its  volume  the  inconceivable 
amount  of  338,000,000,000,000  cubic  miles,  re- 
spectively 119  times  and  1300  times  the  surface 
and  volume  of  the  Earth.  It  is  greater  than  all 
the  other  planets  rolled  into  one. 

Its  mean  distance  from  the  Sun  is  483,000,000 
miles.  Its  orbit  is  considerably  more  elliptical 
than  that  of  the  Earth,  and  Jupiter's  solar  dis- 
tance varies  from  about  462,000,000  miles  to  over 
500,000,000. 

Its  distance  from  the  Earth,  when  nearest,  is 
61 


62      TKe  Essence  of  -Astronomy 

approximately  370,000,000  miles,  and,  when 
farthest  away,  nearly  600,000,000  miles.  Its 
average  nearest  and  farthest  distances  during  the 
year  are,  however,  about  390,000,000  miles  and 
580,000,000  miles. 

Jupiter  revolves  about  the  Sun  in  n  years, 
10  months,  and  17  days. 

It  rotates  on  its  axis  more  swiftly  than  any  of 
the  other  planets.  Its  time  of  rotation  is  about 
9  hours  55  minutes.  This  time  can  be  given 
only  approximately,  because,  like  the  Sun, 
Jupiter's  surface  rotates  faster  at  the  equator 
than  near  the  poles.  This  rapid  rotation  carries 
a  point  on  the  equator  of  the  planet  at  a  speed  of 
nearly  26,000  miles  an  hour,  a  spot  on  the  Earth's 
equator  traveling  slightly  more  than  1000  miles 
an  hour.  This  short  day  of  Jupiter  makes  its 
year  include  10,455  °f  them. 

The  axis  of  Jupiter  is  almost  perpendicular  to 
the  plane  of  its  orbit,  the  inclination  being  only 
3  degrees,  so  it,  too,  has  no  seasons  as  we  know 
them. 

Its  mean  speed  in  its  orbit  is  about  8  miles  a 
second. 

Its  mass  is  slightly  more  than  316  times  that 
of  the  Earth,  but  its  density  is  much  less,  being 


Jvipiter  63 

scarcely  one  fourth.  The  average  attraction  for 
bodies  at  its  surface  is  nearly  2§  times  that 
of  our  planet,  but,  because  of  the  rapid  rotation 
and  the  consequent  high  centrifugal  force,  this 
attraction  varies  almost  20  per  cent,  between  the 
poles  and  the  equator.  A  body  weighing  here 
200  pounds  would  weigh  at  Jupiter's  equator 
about  500  pounds,  and  at  one  of  his  poles  about 
600  pounds. 

It  has  a  very  extensive  atmosphere,  and  it  is 
probable  that  nearly  all  the  surface  markings  we 
see  with  a  telescope  on  Jupiter  are  atmospheric, 
and  not  on  the  true  surface  of  the  planet.  For 
that  matter,  it  is  very  doubtful  whether  there 
is  anything  solid  about  it.  It  would  seem  to  be  a 
ball  of  almost  liquid  consistency  surrounded  by 
clouds  and  vapor. 

It  receives,  when  nearest  the  Sun,  about  one 
third  as  much  light  and  heat  as  does  the  Earth; 
about  two  fifths  as  much  when  farthest 
away. 

The  reflecting  power  of  the  planet  is  high. 
It  sends  back  62  per  cent,  of  the  light  received. 

Jupiter  shows  no  phases,  being  too  far  outside 
our  orbit. 

Its  orbit  is  inclined  to  the  Earth's  less  than 


64      TKe  Essence  of  Astronomy 


any  other  planet's  except  Neptune's,  the  inclin- 
ation being  barely  more  than  1%  degrees. 

MOONS 

Jupiter  has  eight  moons  which  have  been 
discovered.  The  method  of  searching  by  pho- 
tography is  so  nearly  perfect  now  that  it  is  ex- 
tremely doubtful  that  more  will  be  found. 

The  moons  are  usually  known  by  Roman 
numerals.  The  original  four  bear  names  also. 
Their  dimensions,  orbital  times,  and.  distances 
from  them  are: 


Name  or  Number 

Mean  Dist. 
from  Jup. 

Periods  of  Re  vol. 
days  hrs.  min.  sec. 

Diameter 
in  miles 

V 

112,500 

o    ii    57   27 

?(very 

small) 

KIo) 
II  (Europa) 
III  (Ganymede) 
IV  (Calypso) 

26l,OOO 
415,000 
664,000 
I,l67,OOO 

i    18    27    36 
3    13    17    54 
7      3    59    35 
16    18     5      7 

2500 

2IOO 
3560 
2960 

VI 

7,IOO,OOO 

8^4  months 

?(very 

small) 

VII 

7,400,000 

9 

?  "  « 

VIII 

l6,OOO,OOO 

26 

?  "  " 

It  will  be  seen  that  the  system  is  enormous. 
These   moons   apparently   always   turn    the 
same  side  to  Jupiter. 

Their  orbital  speeds  vary  according  to  their 


JUPITER,     1910 

Note  the  distinctly  oval  form  of  the  planet  and  the 

"  belts  " 
From  a  photograph  taken  at  the  Lowell  Observatory 


Jxipiter  65 

distances  from  their  primary.  The  nearest 
traveling  at  about  15  miles  per  second,  the 
farthest  at  about  1.3  miles  per  second. 

Their  reflective  power  varies  and  there  seems 
a  slight  difference  in  color. 

Their  orbits  are  nearly  circular,  and  those  of 
the  inner  ones  are  almost  in  the  plane  of  the 
planet's  equator. 

The  orbits  of  the  outer  ones  are  inclined  at 
over  30  degrees.  The  larger  ones  seem  to  have 
some  atmosphere. 

To  the  telescopist  with  a  small  instrument 
these  moons  are  of  great  interest.  The  changes 
in  position  due  to  their  revolutions  can  be 
detected  in  but  a  few  minutes'  observation,  and 
their  transits,  as  they  pass  directly  between 
Jupiter  and  the  Earth,  their  occultations  as 
they  pass  behind  him,  and  their  eclipses  as  they 
disappear  in  his  shadow,  are  among  the  most 
easily  observed  celestial  phenomena. 

It  was  through  the  discrepancy  between  the 
predicted  and  observed  times  of  the  eclipses  of 
these  moons  that  it  became  known  that  light 
required  time  to  travel. 

Only  four  of  these  moons  may  be  seen  except 
by  the  aid  of  the  great  telescopes.  There  have 


66      XKe  Essence  of  Astronomy 

been  partially  substantiated  cases  of  these  four 
being  seen  with  the  naked  eye.  These  moons 
offer  added  interest  as  being  the  first  celestial 
objects  discovered  by  Galileo  with  the  first 
telescopic  aid. 

The  physical  condition  of  Jupiter  is  un- 
questionably liquid  or  plastic.  As  stated,  the 
markings  seen  are  almost  certainly  atmospheric 
ones,  where  great  "trade  winds"  appear  as  the 
dark  bands  or  "belts"  which  characterize  the 
planet.  Great  semi-permanent  markings,  such 
as  the  famous  Great  Red  Spot,  break  out  from 
time  to  time,  but  the  invariable  fading  of  such 
demonstrates  that,  almost  certainly,  we  do  not 
view  a  solid  surface.  In  the  larger  telescopes 
the  bands,  and,  in  fact,  all  the  surface  show  a 
wonderful  range  of  detail  and  color. 

It  is  suggested  that  Jupiter  is  still  hot  enough 
to  give  forth  a  light  of  its  own.  This  is  doubtful, 
but  there  is  no  doubt  that  it  still  is  very  hot. 

Life  on  Jupiter  itself  is  at  present  entirely  out 
of  the  question,  but  there  is  a  very  doubtful 
possibility  of  such  existing  upon  some  of  the 
larger  moons.  However,  at  that  tremendous 
distance  from  the  Sun,  it  would  seem  necessary 


Jxipiter  67 

for  Jupiter  to  be  hotter  than  is  now  supposed,  if 
the  planet  is  to  radiate  to  them  enough  heat  to 
compensate  for  the  feebleness  of  the  solar  rays 
at  that  distance,  and  maintain  a  temperature 
sufficiently  high  to  support  life. 

Jupiter,  when  viewed  from  the  Earth,  with 
the  naked  eye,  shines  almost  as  brilliantly  as 
Venus,  and  because  of  its  comparatively  slow 
orbital  motion,  remains  for  months  each  year  as 
a  splendid  morning  or  evening  star. 


CHAPTER   X 

SATURN 

SATURN  is  the  second  largest  of  the  planets, 
and  is  the  sixth  in  distance  from  the  Sun. 

Its  mean  diameter  is  about  73,000  miles, 
more  than  9  times  that  of  the  Earth.  The 
polar  flattening  is  very  great,  nearly  r0,  and 
its  polar  diameter  is  only  68,000  miles,  while 
its  equatorial  diameter  is  about  75,000  miles. 
Its  surface  is  about  84  times  that  of  the  Earth, 
and  its  volume  770  times. 

Its  mean  distance  from  the  Sun  is  approxi- 
mately 886,000,000  miles,  or  9)^  times  as  much 
as  the  Earth's.  The  eccentricity  of  its  orbit  is 
considerable,  and,  in  consequence,  the  variation 
in  its  solar  distance  is  nearly  100,000,000  miles. 

Its  distance  from  the  Earth  ranges  from  774,- 
000,000  miles,  when  at  its  nearest,  to  1,028,000,- 
ooo  miles,  when  farthest  away. 

It  revolves  about  the  Sun  in  about  29^  years. 

Its  rotation  upon  its  axis  is  almost  as  swift  as 
68 


Saturn  69 

that  of  Jupiter,  being  nearly  10  hours  14^  min- 
utes. It  is  the  speed  of  this  rotation,  combined 
with  the  small  density  of  the  planet,  that  causes 
the  great  distortion  of  its  shape. 

Its  axis  is  inclined  to  the  plane  of  its  orbit 
nearly  27  degrees,  or  more  than  the  Earth's. 
This  produces  seasons  as  we  know  them. 

Saturn  moves  in  its  orbit  around  the  Sun  at  a 
mean  speed  of  6  miles  a  second. 

The  density  of  this  great  planet  is  surprisingly 
small.  It  has  been  very  accurately  figured  and 
proves  to  be  only  about  one  eighth  that  of  the 
Earth  or  less  than  that  of  water.  Saturn  would 
float  comfortably  in  an  ocean  great  enough  to 
hold  it.  The  mass  is  about  95  times  that  of 
the  Earth,  and  less  than  one  third  that  of 
Jupiter.  Its  mean  attraction  for  bodies  at  its 
surface  is  therefore,  in  spite  of  its  great  size, 
only  about  1.2  times  that  of  the  Earth.  At  the 
equator,  because  of  the  great  centrifugal  force 
counteracting  the  force  of  gravity,  it  is  less  than 
that  of  the  Earth.  The  variation  between  its 
attraction  at  its  poles  and  at  its  equator  is 
nearly  25  per  cent. 

Saturn  has  an  atmosphere  apparently  much 
like  that  of  Jupiter,  and,  as  we  see  in  a  telescope, 


7°     THe  Essence  of  -Astronomy 

similar  bands  and  belts.  It  shows,  at  each  pole, 
moreover,  curious  caps  of  faint  olive  green. 
These  caps  are  not  constant,  but  are  usually 
visible.  Just  what  they  are  is  not  known. 

It  receives  from  the  Sun  on  the  average  only 
about  TQ-Q  as  much  light  and  heat  as  the  Earth. 
This  amount  varies,  of  course,  with  its  distance 
from  the  Sun,  but  it  is  always  pitifully  small. 

Its  reflective  power  is  high,  being  about  that  of 
Venus.  It  throws  back  about  72  per  cent,  of  the 
light  received. 

It  is  too  far  beyond  the  orbit  of  the  Earth 
to  show  phases. 

The  orbit  of  Saturn  is  inclined  to  that  of  the 
Earth  about  two  and  one  half  degrees. 

RINGS 

Saturn  is  unique  in  the  Solar  System  in  the 
possession  of  a  series  of  vast  encircling  rings 
which  lie  in  the  plane  of  its  rotation.  Because 
of  the  inclination  of  Saturn's  axis  to  the  plane  of 
its  orbit,  it  sometimes  presents  one  pole  and 
then  the  other  to  the  Sun,  as  does  the  Earth. 
The  latter,  compared  to  Saturn,  is  so  near  the 
Sun  that  we  view  the  planet  almost  as  if  we  were 
on  the  Sun  itself.  The  result  is  that  we,  at 


SATURN 

December  23,  IQI2.     Note  the  dark  cap,  the  belts,  the 
divisions  in  the  rings,  and  the  oval  form  of 

the  planet 

From  a  photograph  by  E.  C.  Slipher,  taken  at  the  Lowell 
Observatory 


Saturn  71 

times,  see  the  north  surface  of  the  rings,  and, 
about  fifteen  years  later  (one  half  of  Saturn's 
time  of  revolution  about  the  Sun)  the  south 
surface,  while  half  way  between  we  see  them 
presented  edgewise,  when  they  are  visible  only 
through  the  most  powerful  telescopes,  and 
appear  as  a  sort  of  luminous  needle  running 
through  the  planet.  It  is  because  of  this  change 
in  appearance,  as  seen  from  the  Earth,  that 
Galileo  only  nearly  discovered  them.  He  saw 
them  when  he  first  turned  his  telescope  on 
Saturn,  but  as  the  planet  was  approaching  the 
position  when  the  rings  disappear  he  saw  them 
but  faintly,  and  then  seemed  to  see  them  gradu- 
ally vanish.  This  gave  him  the  impression  that 
they  were  illusionary,  and,  in  disgust,  he  never 
studied  the  planet  again.  In  1655,  Huyghens 
discovered  their  real  existence  and  shape. 

There  are  three  concentric  rings  in  all.  The 
outer  one  has  a  diameter  of  about  173,000  miles. 
Its  breadth  is  about  10,000  miles.  There  is  a 
division  space  of  about  1000  miles  between 
that  and  the  next.  The  second  ring  is  quite 
the  broadest,  being  nearly  17,000  across.  The 
third  is  called  the  "cr£pe  ring"  from  its  dusky 
appearance.  It  is  only  visible  in  the  great 


72      TKe  Essence  of  Astronomy 

telescopes.  It  is  semi-transparent.  Between 
the  inner  edge  of  the  third  ring  and  the  surface 
of  the  planet  is  a  space  of  about  9000  miles. 

These  rings  are  extremely  thin  in  proportion 
to  their  diameters,  the  paper  upon  which  this  is 
printed  is  thicker  in  proportion  to  its  size  than 
they  are.  The  thickness  of  the  rings  is  estimated 
at  about  100  miles. 

The  rings  were  first  considered  solid.  It  has 
now  been  demonstrated  that  they  must  be 
composed  of  countless  tiny  "moonlets, "  possibly 
only  infinitely  small  particles,  each  pursuing  its 
own  orbit  about  the  planet.  When  the  rings 
are  seen  edgewise  there  appear  certain  thick- 
enings or  knots  as  if  clusters  of  these  particles 
had  formed. 

It  has  not  been  settled  whether  these  rings 
are  a  permanency  or  whether  they  will  eventu- 
ally be  dispersed.  Recently  an  extremely  faint 
outer  ring  has  been  observed  similar  to  the 
"cr£pe"  ring.  This  would  seem  to  suggest 
almost  the  probability  of  an  eventual  breaking 
up  of  this  marvelous  system,  these  two  " crepe" 
rings  being  possibly  the  beginning  of  the  dis- 
integration. 


Saturn 

MOONS 


73 


Saturn,  of  all  of  the  planets,  possesses  the 
greatest  retinue  of  satellites.  No  less  than  ten 
moons  are  now  known  to  encircle  it.  Their  di- 
mensions, orbital  times,  and  distances  from  the 
planet  are: 


Periods  of  Re  vol. 

Mean  Dist. 

Diameters 

fr.  Saturn 

days 

hrs. 

min. 

I.  Mimas 

II7,OOO 

22 

36 

600 

II.  Enceladus 
III.  Tethys 

157,000 
l86,OOO 

I 
I 

8 

21 

54 
18 

800 
1,100 

IV.  Dione 

238,OOO 

2 

17 

42 

1,200 

V.  Rhea 

332,000 

4 

12 

30 

1,500 

VI.  Titan 
X.  Themis 

771,000 
9O6,OOO 

15 
20 

20(?) 

is 
o 

3,500 

3o(?) 

VII.  Hyperion 
VIII.  Japetus 

943,000 
2,225,000 

21 

79 

7 

21 

36 
6 

500 
2,000 

IX.  Phoebe 

8,000,000 

580 

2 

54 

40(?) 

As  in  the  case  with  Jupiter,  it  will  be  seen 
that  the  size  of  the  system  is  enormous.  Jape- 
tus has  a  period  of  nearly  that  of  Mercury  while 
the  revolution  of  Phcebe  requires  over  eighteen 
months. 

Japetus  shows  a  distinct  difference  in  bril- 
liancy during  its  orbit,  being  always  brighter 


74     THe  Essence  of  Astronomy 

when  west  of  the  planet  than  when  east  of  it. 
This  shows  that  it  follows  the  general  rule  of 
satellites  and  turns  always  the  same  side  to  its 
primary — one  half  of  its  surface  having  a  higher 
reflecting  power  than  the  other. 

The  motion  of  Phoebe  is  unique — its  revolu- 
tion is  retrograde,  that  is,  opposite  to  the  rotation 
of  its  primary.  Prof.  W.  H.  Pickering,  the  dis- 
coverer, suggests  two  possible  explanations  for 
this.  One  is  "that  Phoebe  was  originally  simply 
a  comet  which  was  captured  by  Saturn.  In  that 
case,  if  the  comet  passed  on  one  side  of  Saturn, 
its  orbital  motion  would  be  direct,  if  on  the 
other,  retrograde."  The  other  is  "that  at  the 
remote  time  when  it  [Phoebe]  was  formed,  and 
when  the  mass  of  Saturn  filled  the  whole  orbit 
of  its  satellite,  Saturn  was  then  revolving  in 
a  retrograde  direction.  During  the  ages  that 
elapsed  before  the  next  satellite,  Japetus,  came 
into  existence,  Saturn  had  changed  the  plane  of 
its  rotation  [or  turned  over  part  way,  (compare 
with  the  theory  regarding  Uranus  and  Neptune 
in  the  following  chapters)]  so  that  the  satellite's 
motion  was  direct." 

Themis,  the  latest  addition  to  the  family,  is 
hopelessly  beyond  the  range  of  direct  vision 


Saturn  75 

with  even  the  greatest  of  our  telescopes.  It, 
like  Phoebe,  was  discovered  by  photography. 

The  orbits  of  the  five  inner  satellites  are  almost 
circular;  those  of  the  outer  ones  are  much  more 
elliptical.  Japetus  and  Phoebe  have  orbits 
inclined  to  the  plane  of  the  rings  about  ten 
degrees  and  six  degrees  respectively.  The  other 
satellites  all  move  almost  exactly  in  the  plane  of 
the  rings.  The  orbital  speed  of  the  moons  ranges 
from  about  9  miles  per  second  for  the  inner  one 
to  about  one  mile  a  second  for  the  outermost. 

It  is  not  improbable  that,  in  the  great  gap 
between  Titan  and  Japetus,  other  satellites 
beside  Hyperion  and  Themis  may  be  discovered. 
Such,  however,  must  be  extremely  small. 

Saturn  was  the  outermost  planet  known  to  the 
ancients,  and  shines  as  a  bright  star  with  a 
steady  golden  light. 

Like  Jupiter  it  is  probably  very  hot.  There 
has  been  some  question  whether  Saturn,  too, 
shines  partly  by  its  own  light,  but  this  seems 
decided  in  the  negative.  That  there  is  no  solid 
matter  in  its  composition  is  certain,  the  planet's 
low  density  and  great  equatorial  bulge  put  that 
beyond  doubt. 


76      THe  Essence  of  Astronomy 

The  surface  markings  seen  in  a  telescope,  are 
without  doubt,  like  those  of  Jupiter,  merely  at- 
mospheric; great  trade  winds  of  dense  vapors, — 
the  similarity  between  the  two  planets  being 
great. 

Life  on  Saturn  is  impossible,  as  we  know  life, 
and  that  its  moons  could  support  such  seems 
most  improbable.  Unless  Saturn  radiates  far 
more  heat  than  appears  reasonable,  the  moons 
must  be  frozen,  dead  globes. 

To  the  "sight-seer"  through  a  telescope, 
Saturn  and  his  system  proves  a  most  surprisingly 
interesting  and  beautiful  sight. 


Note.— Dr.  Percival  Lowell  has  just  shown  the  author 
some  marvellous  photographs  of  the  inner  satellites  of 
Saturn.  These  photographs,  taken  recently  at  the  Lowell 
Observatory,  clearly  prove  that  Tethys  and  Dione  vary 
regularly  in  brilliancy,  according  to  their  orbital  positions; 
demonstrating  that  they,  too,  turn  always  the  same  face 
to  the  planet. 


CHAPTER  XI 

URANUS 

URANUS  is  the  fourth  from  largest  of  the 
planets,  and  is  the  seventh  in  order  from  the  Sun. 

Its  mean  diameter  is  somewhat  in  question 
being  estimated  from  28,500  to  35,000  miles. 
Probably  about  32,000  miles  is  nearly  correct. 
Its  surface  is  roughly  sixteen  times  that  of  the 
Earth,  and  its  volume  about  65  times.  It 
shows  a  distinctly  oval  disk,  the  ellipticity  being 
about  one  fourteenth  of  the  diameter. 

Its  mean  distance  from  the  Sun  is  about  19 
times  that  of  the  Earth,  or  nearly  i  ,800,000,000 
miles.  Its  orbit  is  not  quite  so  elliptical  as 
that  of  Jupiter,  but  its  distance  from  the  Sun 
varies  about  70,000,000  miles  in  the  course  of  one 
revolution. 

Its  mean  distance  from  the  Earth  varies  be- 
tween about  1,893,00,000  miles  and  1,707,000,- 
ooo  miles. 

77 


78      XKe  Essence  of  Astronomy 

It  revolves  about  the  Sun  in  slightly  over  84 
years. 

Its  time  of  rotation  upon  its  axis  is  not  known, 
owing  both  to  its  great  distance  of  its  peculiar 
axial  inclination,  and  a  lack  of  definite  spots  or 
separated  markings  upon  its  surface. 

Its  axis  is  inclined  to  the  plane  of  its  orbit  to 
such  a  degree  that  Uranus  may  be  compared  to 
a  top  rolling  on  its  side,  not  standing  on  its 
point.  This  is  not  definitely  agreed  upon  but 
seems  logical  since  we  look  at  the  revolution  of  its 
satellites  not  as  with  the  inner  planets — edge 
on — but  as  if  we  were  watching  a  wheel  revolve, 
the  axis  being  pointed  at  us;  and  practically  all 
satellites  revolve  fairly  close  to  the  plane  of  the 
rotation  of  their  primary. 

Uranus  moves  in  its  orbit  with  the  slow  plane- 
tary speed  of  about  4.3/5  miles  per  second.  , 

Its  mass  is  14^  times  that  of  the  Earth,  but 
its  density  only  slightly  more  than  one  fifth.  Its 
attraction  therefore  for  bodies  at  its  surface  is, 
in  spite  of  its  size,  somewhat  less  than  the  Earth's. 
A  body  weighing  200  Ibs.  here  would  weigh  upon 
Uranus  about  180  Ibs. 

It  has  an  extensive  atmosphere  which  the 
spectroscope  tells  us  contains  some  substande 


Uranus 


79 


not  yet  identified  on  the  Earth.  It  is  probably 
this  substance  which  accounts  for  the  distinct 
greenish  tinge  of  the  planet's  light. 

It  receives  from  the  Sun  on  an  average  of 
barely  more  than  5^0"  of  the  light  and  heat 
reaching  the  Earth. 

Its  reflective  power  is  high,  somewhat  more 
than  Jupiter's.  It  sends  back  64  per  cent,  of 
the  light  striking  it. 

It,  of  course,  shows  no  phases. 

Its  orbit  lies  almost  in  the  plane  of  the  Earth's, 
more  nearly  so  than  that  of  any  other  planet. 

MOONS 


Dist.  from 

Periods  of  Re  vol. 

Diameter 

Planet 

day 

hour 

min. 

(Est.  only) 

Ariel 

I2O,OOO 

2 

12 

30 

600 

Umbriel 

167,000 

4 

3 

30 

500 

Titania 
Oberon 

293,000 
365,000 

8 
13 

16 
ii 

56 

I20O 
IOOO 

It  has  been  considered  that  they  revolve  back- 
wards like  the  ninth  satellite  of  Saturn.  It  is 
now  almost  certain  that  they  revolve  forward; 
that  is  in  the  same  direction  as  the  planet  rotates. 
This  has  been  open  to  question,  as  explained  in 
the  last  paragraph  of  this  chapter. 


8o     THe  Essence  of  Astronomy 

Uranus  was  the  first  planet  ever  discovered. 
All  the  others  have  been  known  since  history 
began.  It  was  found  accidently  by  Sir  William 
Herschel,  in  1781,  while  "sweeping"  the  heavens 
for  what  he  might  find  with  a  telescope  of  his  own 
manufacture.  At  first,  though  seeing  immedi- 
ately that  it  was  not  a  star,  its  true  nature  did 
not  occur  to  him.  For  some  time  it  was  studied 
as  a  comet,  and  not  until  almost  a  year  later  was 
it  recognized  as  a  new  planet. 

It  shines  as  a  faint  star,  of  greenish  tint,  just 
visible  to  good  eyes  under  clear  conditions. 

It  shows  faint  bands  or  "belts"  like  those  on 
Jupiter  or  Saturn. 

The  inclination  of  the  axis  of  Uranus  is  not 
entirely  agreed  upon.  If  it  is  less  than  90 
degrees,  the  satellites  have  retrograde  revolu- 
tions, but  if  more  than  90  degrees  the  satellites 
revolve  direct.  This  would  seem  more  logical. 
Professor  Lowell  believes  that  Uranus  will, 
through  the  course  of  ages,  turn  over  (as  will 
a  gyroscope  when  rotated  one  way  and  swung 
in  the  opposite  direction) ,  until  its  axis  is  eventu- 
ally perpendicular  to  the  plane  of  its  orbit ;  that  it 
started  in  an  inverted  position  and  has  not  quite 
reached  the  half-way  mark  in  its  attempt  to 


Uranus  81 

conform  to  the  proper  planetary  behavior.  His 
theory  is  not  universally  accepted,  but  it  is  the 
most  logical  explanation  of  the  curious  position 
of  Uranus,  and  also  of  that  of  Neptune.  This 
idea  is  in  accord  with  one  of  the  explanations 
Professor  Pickering  gives  for  the  retrograde  re- 
volution of  Saturn's  outermost  satellite. 

6 


CHAPTER   XII 

NEPTUNE 

As  far  as  is  now  known,  Neptune  is  at  the 
frontier  of  the  Solar  System.  It  is  the  third  from 
the  largest  of  the  planets  and  the  eighth  in  dis- 
tance from  the  Sun. 

Its  diameter  is  somewhat  more  than  that  of 
Uranus,  being  probably  about  33,000  to  34,000 
miles.  Some  measurements  make  it  as  low  as 
28,500.  Its  surface  is  about  19  times  that 
of  the  Earth  and  its  volume  85  to  90  times.  It 
shows  no  measurable  polar  flattening. 

Its  mean  distance  from  the  Sun  is  2,800,- 
000,000  miles,  and  even  though  the  planet's 
orbit  is  nearly  circular,  the  diameter  of  its  orbit 
is  so  great  that  even  a  small  ellipticity  varies  its 
distance  from  the  Sun  nearly  50,000,000  miles 
during  a  revolution. 

Neptune  requires  164  of  our  years  to  make  one 
revolution  about  the  Sun. 

The  rotation  time  is  in  doubt.  It  is  even  more 
to 


Neptune  83 

difficult  to  measure  than  that  of  Uranus.  Nep- 
tune probably  rotates  somewhat  slower  than 
the  latter.  The  density  of  these  two  planets  is 
about  the  same,  and,  while  Uranus  shows  a 
"bulge"  at  its  equator,  due  to  the  centrifugal 
force  of  rotation,  Neptune,  as  said  above,  does 
not  show  enough  to  measure.  Its  rotation  is 
usually  spoken  of  as  retrograde,  or  backward. 

Its  axis  is  inclined  to  the  plane  of  its  orbit 
either  35  degrees  or  125  degrees.  If  the  former, 
the  reason  for  the  retrograde  rotation  is  difficult 
to  explain,  if  the  latter,  Professor  Loweirs 
theory  may  fit  the  case.  With  Neptune  he 
believes  that,  owing  to  its  much  greater  distance 
from  the  Sun,  its  slower  orbital  speed,  as  well  as 
probably  slower  rotation  than  Uranus,  the  effort 
to  "right  itself"  is  much  more  feeble,  and  will 
take  longer  time.  But,  like  Uranus,  it  is  doing 
its  best  to  turn  over,  and  so  conform  to  the 
general  custom  of  properly  behaved  planets  by 
rotating  the  right  way  round.  As  mentioned 
before,  this  theory  is  not  generally  accepted.  It 
certainly  is  logical,  however,  and  gives  an  ex- 
planation of  a  fact  for  which  as  yet  no  other 
apparently  sound  reason  has  been  advanced. 

Neptune,  being  the  farthest  away  from  the 


84      THe  Essence  of  Astronomy 

Sun,  moves  more  slowly  than  any  of  the  planets, 
its  orbital  speed  being  only  about  ^A,  miles  per 
second. 

Its  mass  is  about  17  times  that  of  the  Earth, 
but  its  density  only  about  one  fifth.  Its  attrac- 
tion for  bodies  at  its  surface,  therefore,  is  less 
than  that  of  the  Earth,  and  slightly  more  than 
that  of  Venus.  A  body  weighing  200  Ibs.  here 
would  on  Neptune  weigh  about  175  Ibs. 

It  has  a  very  extensive  atmosphere.  The 
more  carefully  this  is  studied  the  farther  it  is 
found  to  reach. 

Neptune  receives  from  the  Sun  only  a  wretch- 
edly small  proportionate  share  of  light  and  heat, 
about  rsVo  of  that  which  warms  and  illuminates 
the  Earth.  Unless,  therefore,  the  planet  be 
still  intrinsically  hot  its  temperature  must  be 
low  beyond  description.  Do  not  think,  however, 
that  there  is  no  real  daylight  on  Neptune.  The 
Sun,  though  appearing  so  small  as  not  to  be 
seen  as  a  real  disk  by  the  naked  eye  nevertheless 
gives  as  much  light  as  about  700  of  our  full 
moons.  "As  seen  from  Neptune,  the  Sun  would 
look  like  a  brilliant  electric  arc-lamp  at  a  dis- 
tance of  a  few  yards. " 

The  reflective  power  of  the  planet  is  less  than 


Neptune  85 

that  of  Saturn.  It  throws  back  somewhat  more 
than  half  the  light  received. 

It  shows  no  phases. 

Its  orbit  is  inclined  to  that  of  the  Earth  about 
one  and  three  quarters  degrees. 

Neptune,  like  the  Earth,  has  apparently  only 
one  moon.  This  is  as  yet  unnamed.  Its  di- 
ameter is  estimated,  by  measuring  the  light  it 
reflects,  as  about  2000  miles,  or  nearly  the  size 
of  our  own  Moon. 

The  satellite's  distance  from  the  planet  is  also 
about  the  same  as  the  Moon's  from  us — 223,000 
miles. 

It  revolves  about  Neptune  in  5  days  21  hours, 
and  moves  as  Neptune  rotates — that  is,  in  an 
apparently  backward  direction. 

Of  its  rotation  nothing  is  known.  Presum- 
ably like  most  moons  it  always  presents  the  same 
side  to  its  primary. 

Its  orbit,  like  most  satellites,  lies  close  to  the 
plane  of  the  planet's  equator. 

It  is  visible  only  in  large  telescopes,  and  is  a 
difficult  object  to  see. 

It  was  discovered  within  a  month  of  the  dis- 
covery of  Neptune  itself. 


86     THe  Essence  of  Astronomy 

Neptune's  discovery  in  1846  marked  the  great 
triumph  of  mathematical  astronomy.  Uranus 
had  failed  to  move  in  its  orbit  precisely  as 
predicted,  and  astronomers  were  at  a  loss  to 
explain  the  fault.  The  difference  between  the 
predicted  and  the  actual  positions  was  extremely 
slight  from  a  naked  eye  view-point,  but  that 
there  were  differences  showed  that  something 
was  wrong.  Two  men,  Adams  of  England, 
and  Leverrier  of  France,  independently  took 
up  the  solution  of  the  problem,  and  not  only 
each  arrived  at  the  conclusion  that  a  planet 
exterior  to  Uranus  was  causing  the  perturbations 
of  the  latter,  but  each  computed  this  hypotheti- 
cal planet's  position  with  amazing  accuracy. 
The  observatory  to  which  Leverrier  sent  his 
calculations  was  the  first  to  recognize  the  planet 
within  but  a  very  short  distance  of  where  its 
position  had  been  figured.  For  many  years 
Adams  was  not  given  due  credit  for  his  marvel- 
ous labor,  since,  though  his  calculations  were 
completed  months  before  those  of  Leverrier,  the 
Astronomer  Royal  of  England,  to  whom  Adams 
sent  his  figures,  was  too  methodical  a  man  to 
take  up  the  search  for  this  supposed  planet 
until  it  was  too  late  to  be  first.  It  may  be  said 


Neptvine  87 

that  the  two  men,  Adams  and  Leverrier  fairly 
divide  the  great  honor. 

The  surface  of  Neptune  shows  practically  no 
markings  except  a  faint  suggestion  of  a  band 
or  "belt"  similar  to  those  of  the  other  larger 
planets.  Its  light  seems  to  have  a  faint  tinge 
of  the  same  green  shown  by  Uranus. 

It  will  be  noticed  that  Neptune  and  Uranus 
are  much  alike  in  many  respects ;  as  also  are  the 
Earth  and  Venus. 

Life  on  Neptune  seems,  of  course,  utterly 
beyond  possibility. 


CHAPTER  XIII 

COMETS 

COMETS  are  the  "fiery-haired"  stars  of  the 
ancients,  the  word  comet  being  derived  from  the 
Greek  XO^TY)?  (long-haired).  From  time  im- 
memorial, they  have  been  regarded  by  the 
ignorant  as  portents  of  evil,  presaging  the  fall 
of  empires,  the  death  of  kings,  the  coming  of 
pestilence,  the  approach  of  war,  etc.  Science, 
in  proving  the  fallacy  of  such  ideas,  has  rendered 
a  great  benefit  to  mankind. 

Comets  may  be  divided  into  several  classes: 

Naked-eye  comets  and  telescopic  comets,  of 
which  the  latter  are  by  far  the  more  numerous. 
It  is  size  rather  than  distance  from  the  Earth 
that  determines  the  inclusion  of  a  comet  in  one 
group  or  in  the  other. 

Periodic  comets   and   parabolic  or  hyperbolic 

comets.     Here  the  difference  is  in  the  apparent 

shape  of  the  orbit.     Periodic  comets  revolve 

about  the  Sun  in  elliptical  orbits,  as  do  the 

88 


Comets  89 

planets,  but  in  ellipses  of  great  eccentricity,  or 
elongation.  Parabolic  and  hyperbolic  comets 
are  those  which  apparently  (note  the  apparently) 
pass  the  Sun  in  curves  that  are  not  closed,  that 
is  in  curves  which  never  return  upon  themselves, 
and  which  would  therefore  allow  a  comet  fol- 
lowing such  a  curve  to  pass  the  Sun  but  once. 
It  is  an  open  question  whether  any  comet  moves 
in  such  an  orbit.  None  are  known  certainly  to 
do  so;  many  are  known  to  move  in  ellipses;  no 
authority  will  express  absolute  belief  in  the 
parabolic  or  hyperbolic  orbit,  and  several  are 
willing  to  state  definitely  that  such  an  orbit  does 
not  exist.  The  balance  seems,  therefore,  to  be 
against  the  "open"  orbit  and  in  favor  of  the 
eventual  return  of  all  comets,  unless  they  are 
destroyed  or  disintegrate.  The  reason  this 
question  is  so  difficult  to  decide  is  that  the  total 
path  of  a  celestial  body  may  be  constructed  only 
by  measuring  and  plotting  the  curve  of  that 
part  of  the  path  visible  to  us.  In  the  large 
cometary  orbits  this  is  so  small  proportionately 
that  it  is  almost  impossible  to  decide  whether 
we  have  seen  part  of  a  tremendously  eccentric, 
or  elongated  ellipse,  or  part  of  a  parabola  or 
hyperbola. 


90      TKe  Essence  of  Astronomy 

Short-period  and  Long-period  comets.  In  this 
case  the  difference  is  only  in  the  time  of  revolu- 
tion about  the  Sun.  This  time  varies  from  two 
or  three  years  up  to  several  thousands  and  if  the 
apparent  parabolic  comets  are  indeed  periodic, 
up  to  perhaps  scores  of  thousands. 

A  comet  is  usually  composed  of  three  main 
parts;  the  nucleus,  the  coma,  and  the  tail. 

The  nucleus  is  a  bright  star-like  point  of  light, 
which  under  a  telescope  shows  an  ill-defined 
disk.  In  large  comets  it  is  sometimes  many 
hundreds  or  even  many  thousands  of  miles  in 
diameter. 

The  coma  is  a  foggy  mass  surrounding  the 
nucleus.  The  two  together  look  like  a  star 
shining  through  a  haze.  They  comprise  the 
head  of  the  comet. 

The  tail,  when  such  exists,  is  an  extension  of 
the  coma.  This  sometimes  is  many  millions 
of  miles  in  length.  It  always  points  away  from 
the  Sun. 

Comets  are  almost  certainly  composed  of  gases 
and  close  swarms  of  meteors,  "or  small  detached 
masses  of  matter.  These  masses  are  so  small 
and  so  numerous  that  they  look  like  a  cloud,  and 
the  light  which  they  reflect  to  our  eyes  has  the 


™     8, 


"2  ^  % 


3 1 


o    ~      5 

O      ~          g 


Comets  91 

milky  appearance  peculiar  to  a  comet.  On  this 
theory,  a  telescopic  comet  without  a  nucleus  is 
simply  a  cloud  of  these  minute  bodies.  The 
nucleus  of  the  bright  comets  may  either  be  a 
more  condensed  mass  of  such  bodies,  or  it  may 
be  a  solid  or  liquid  body  itself." — (Newcomb). 
The  tail  is  not  a  permanent  appendage,  but 
forms  as  described  below. 

When  the  comet  is  far  from  the  Sun,  it  shows 
no  tail  and  often  no  nucleus,  and  appears  only 
as  a  patch  of  cloudy  light.  As  it  approaches  the 
Sun,  it  grows  brighter  and  a  nucleus  generally 
forms  at  its  brightest  point.  Then  the  coma  be- 
gins to  extend  away  from  the  Sun  and  lengthens 
gradually  and  slowly  at  first,  into  the  tail.  In 
bright  comets,  envelopes  or  bows  of  light  form 
about  the  nucleus  on  the  side  toward  the  Sun,  and 
extend  to  mingle  with  the  tail.  As  the  comet 
passes  from  the  Sun  on  its  outward  journey, 
these  changes  occur  in  reverse  order,  and  the 
comet  reverts  to  a  hazy  patch  of  milky  light 
which  gradually  is  lost  to  view. 

Part  of  the  light  of  a  comet  is  that  of  the  Sun 
reflected,  but  comets  are  self-luminous  when 
near  the  latter.  What  causes  the  action  among 
the  particles  composing  a  comet's  head  to 


92      THe  Essence  of  Astronomy 

produce  this  light  is  not  definitely  known.  It 
certainly  is  connected  in  some  way  with  the  solar 
influence,  and  may  be  the  effect  of  heat  or  of 
some  electrical  action,  or  may  be  the  result  of 
the  Sun's  gravitational  perturbations  among  the 
particles  of  the  head,  causing  them  to  collide 
with  each  other  and  fuse  into  gases,  or  at  least 
to  glow  from  the  heat  of  the  collisions.  It  is 
probably  occasioned  by  a  combination  of  all 
these  factors.  The  actual  light  of  the  Sun  may 
have  effect,  as  it  apparently  has  in  connection 
with  the  formation  of  the  tail. 

The  tail  is  supposedly  formed  by  the  repelling 
action  of  the  light  of  the  Sun  upon  the  tiny 
particles  of  matter  in  the  coma.  It  has  been 
demonstrated  that  light  does  exert  a  physical 
force  of  this  kind.  On  large  bodies  this  pres- 
sure of  light  has  no  appreciable  effect,  as  they 
are  held  by  the  greater  power  of  the  force  of 
gravitation.  Gravitation  acts  on  the  mass  of 
a  body  while  a  pressure  acts  on  the  surface  of  a 
body.  Decrease  the  size  of  a  body  and  the  mass 
decreases  as  the  cube  while  the  surface  decreases 
only  as  the  square.  It  is  possible,  therefore, 
to  reach  a  size  of  body  small  enough  that  the 
power  of  the  light  pressure  will  be  greater  than 


Cornets  93 

that  of  the  gravitational  pull.  It  is  of  particles 
of  this  very  tiny  size  that  the  tail  of  a  comet  is 
composed,  and  they  are  therefore,  driven  away 
from  the  Sun  by  its  light  in  spite  of  the  effort  of 
gravity  to  pull  them  to  the  Sun.  It  is  probable 
that  these  particles  never  rejoin  the  comet's 
head  and  are  left  in  space  scattered  along  the 
comet's  orbit.  Many  of  the  smaller  comets 
never  develop  a  tail. 

The  mass  of  comets  is  relatively  very  small. 
Stars  shine  through  all  parts  of  them,  including 
the  nucleus  at  times ;  and  through  the  tail  with 
certainly  no  appreciable  diminution  of  light. 
This  shows  a  great  lack  of  density.  The  tails 
of  comets  are  certainly  extremely  rarefied,  proba- 
bly more  so  than  the  nearest  approach  to  a 
vacuum  possible  to  obtain  on  the  Earth.  Several 
times,  indeed,  the  Earth  has  passed  through  a 
comet's  tail  without  our  recognizing  the  fact  from 
any  change  in  conditions, — except  possibly  a 
faint  luminosity  of  the  atmosphere.  This  hap- 
pened in  the  case  of  Halley's  comet  in  1910. 

Prophecies  are  often  heard  of  the  dire  fate  cer- 
tain to  befall  the  Earth  through  cometary  col- 
lision. In  the  first  place,  the  mathematical  chance 
of  such  a  collision  is  once  in  about  15,000,000 


94     THe  Essence  of  Astronomy 

years,  and  in  the  second  place  it  is  not  at  all 
certain  that  we  should  suffer  more  than  from  a 
great  meteoric  shower, — the  harmful  effects  of 
which  are  nil.  If  the  particles  forming  the  head 
and  particularly  the  nucleus  of  a  large  comet 
should  strike  the  Earth  squarely,  there  would 
probably  be  some  local  damage  in  the  direct 
line  of  the  collision,  but  this  only  if  these  particles 
were  of  large  enough  size  to  get  through  our  air 
without  being  dissipated  by  the  great  heat  of  the 
friction.  As  a  matter  of  fact,  the  Earth  several 
times  a  year  does  meet  the  remains  of  comets  in 
the  shape  of  meteor  swarms,  and  the  contact  is 
noticed  6y  no  one  except  the  diligent  "star- 
gazer." 

Regarding  the  origin  of  comets  there  has  been 
much  theorizing.  It  seems  probable  that  they 
are,  like  the  meteors,  described  in  the  next 
chapter,  uncollected  fragments  of  the  once 
great  parent  body,  shattered  to  pieces  in  the 
past  ages,  from  whose  parts  the  members  of  the 
Solar  System,  including  the  Sun,  were  formed. 

Comets  differ  from  the  great  bodies  of  the 
System  in  their  lack  of  permanence.  They  break 
up,  causing  the  "remains"  of  which  we  have 
just  spoken. 


Comets  95 

The  one  known  as  Biela's  comet  was  seen  in 
1846  to  split  in  two,  depart  from  the  Sun  with 
two  heads,  and  return  in  1852  widely  separated! 
Since  then  it  has  not  been  seen,  but  in  1872  there 
was  a  meteor  shower  just  as  the  Earth  was  pass- 
ing the  track  of  the  lost  comet,  these  meteors 
unquestionably  being  part  of  what  was  left  of  it. 

There  is  a  certain  curious  relation  between  the 
orbits  of  about  30  comets  and  that  of  the  mighty 
planet  Jupiter,  and  it  is  evident  that  the  planet 
is  responsible  for  the  paths  of  the  comets,  having 
pulled  them  out  of  greater  orbits  by  his  gravita- 
tional attraction  as  they  passed  too  close  to  him, 
and  swung  them  into  shorter  periods.  The  other 
great  planets  have  similar  "families,"  though 
composed  of  fewer  members.  In  1770,  a  bright 
comet  was  discovered  having  a  period  of  only 
five  or  six  years.  This  should,  therefore,  have 
been  seen  before,  and  many  times  since;  and 
yet  it  was  visible  just  that  once.  What  hap- 
pened almost  certainly  is  this ;  coming  from  far 
outside  the  planetary  boundaries  upon  its  sun- 
ward journey,  either  returning  in  a  great  ellipse 
or  passing  in  a  parabola,  it  approached  too  close 
to  Jupiter,  who  whirled  it  into  a  short-period 
orbit.  Upon  its  first  return  in  this  new  path  the 


96     THe  Essence  of  Astronomy 

Earth  was  so  placed  that  the  comet  was  not 
visible;  and  upon  its  passing  outward  it  again 
went  so  near  Jupiter  that  the  latter  threw  it  into 
an  orbit  so  entirely  different  that  it  never  has 
been  seen  since. 

Up  to  the  time  of  Edmund  Halley  in  the  iyth 
century,  all  comets  were  supposed  to  be  visitors 
from  far  space.  It  was  because  of  this,  doubt- 
less, that  so  much  fear  attached  to  their  appear- 
ance. The  great  Newton  showed  in  1680  that 
the  orbit  of  a  comet  then  visible  could  be  either 
a  hyperbola,  a  parabola,  or  an  ellipse.  Halley, 
studying  the  comet  of  1682,  was  struck  by  the 
similarity  of  its  path  with  that  of  the  comets  of 
1607  and  1531.  After  careful  research  and  cal- 
culation, using  Newton's  figures  of  two  years 
previous,  he  arrived  at  the  conclusion  that  they 
were  all  one  and  the  same  comet;  and  he  then 
predicted  its  return  in  1758.  This  prediction 
was  fulfilled,  and  Halley's  modest  plea,  that  if 
it  should  return,  "impartial  posterity  will  not 
refuse  to  acknowledge  that  it  [the  return]  was 
discovered  by  an  Englishman,"  was  met  by 
giving  his  name  to  this  now  most  famous  member 
of  the  cometary  ranks.  Halley's  comet  has 
since  appeared  in  1835  and  1910. 


HALLEY'S    COMET 

Photographed  May  13,  IQIO.     Note  the  peculiar  shape  of  the 
tail  and  the  meteor  trail  crossing  it.      Length  of  tail  50° 

The  round  spot  of  light  is  the  planet  Venus 
From  a  photograph  taken  at  the  Lowell  Observatory 


Comets  97 

The  number  of  comets  is  legion.  Hardly  a 
year  passes  without  the  discovery  of  at  least 
several  small  ones.  The  great  ones,  however, 
are  scarce,  about  20  to  40  per  century,  and  only 
a  small  minority  of  these  are  well  remembered. 
Some  of  them  have  been  so  bright  that  they  have 
been  visible  in  full  daylight,  an  example  of  which 
was  seen  but  a  few  years  ago. 

Some  of  the  more  famous  of  the  comparatively 
recent  comets  are: 

Comet  of  1680.  This  was  of  marvellous  size 
and  brilliancy  and  inspired  such  terror  that  a 
medal  was  distributed  in  Europe  to  quiet  appre- 
hension. A  free  translation  of  the  inscription 
upon  this  medal  is:  "The  star  threatens  evil; 
trust  only!  God  will  make  all  right."  It  was 
this  comet  which  inspired  Newton  to  compute 
its  possible  orbit. 

Comet  of  1682.  Halley's  Comet.  It  is  in- 
teresting particularly  as  the  first  one  to  have 
its  return  predicted.  It  is  the  appearance  of 
this  comet  in  1066  which  is  depicted  upon  the 
famous  Bayeux  Tapestry.  Its  return  in  1910 
was  not  well  viewed  from  the  Northern  Hemi- 
sphere and  the  casual  observer  here  was,  in 
consequence,  disappointed.  In  South  Africa 


98      THe  Essence  of  Astronomy 

and  Australia  the  comet  was  a  most  beautiful 
sight. 

Comet  of  1772.  Biela's  Comet.  A  small  comet 
but  interesting  as  the  first  seen  to  disintegrate, 
as  described  above. 

Comet  of  1811.  Another  very  beautiful  and 
brilliant  comet.  Its  orbit  is  calculated  to  extend 
over  40,000,000,000  of  miles  from  the  Sun.  It 
will  not  return  for  over  1500  years. 

Comet  of  1843.  One  of  the  most  brilliant  of 
all  comets.  It  was  visible  close  to  the  Sun  in 
full  daylight.  Much  fear  was  caused  in  some 
quarters  lest  it  presage  the  end  of  the  world  as 
predicted  for  that  year,  by  William  Miller,  the 
religious  fanatic,  who,  during  the  previous  decade 
constantly  reiterated  this  prophecy.  This  comet 
passed  nearer  to  the  Sun  than  any  other  body 
ever  seen,  and,  as  it  rounded  the  Sun,  moved 
at  the  terrific  speed  of  over  300  miles  per  second. 
It  has  a  period  of  about  500  years. 

Comet  of  1858.  This  is  better  known  as 
Donati's  Comet,  named  after  its  discoverer.  It 
is  the  most  beautiful  on  record,  having  a  very 
brilliant  star-like  nucleus,  and  a  series  of  won- 
derful curving  tails.  From  it  much  valuable 
information  as  to  cometary  composition  was 


Comets  99 

obtained  by  means  of  the  spectroscope,  the 
value  of  which  was  then  beginning  to  be  appre- 
ciated. Its  period  is  about  2000  years. 

Comet  of  1882.  This  was  a  splendid  comet 
with  a  great  tail  shaped  like  a  Turkish  scimitar. 
It  was  of  particular  interest  because  of  a  peculiar 
multiple  nucleus. 

Comet  of  1908.  Moorehouse  (discovered  by). 
A  small  comet  of  great  interest  because  of  some 
amazingly  rapid  and  great  changes  in  its  tail. 
This  tail,  in  the  course  of  a  very  few  hours,  was 
seemingly  twisted  and  torn  as  if  by  external 
agency.  > 


CHAPTER  XIV 


OCCASIONALLY,  on  a  clear  night,  a  long  trail  of 
light  flashes  for  a  moment  across  the  sky.  This 
is  the  well-known  phenomenon  of  the  "falling" — 
or  "shooting-star."  Sometimes,  in  a  tremen- 
dous blaze  and  with  a  great  rushing  sound,  often 
almost  explosive,  a  mass  of  stone  or  metal  hurls 
from  the  sky  to  the  Earth,  is  found,  and  called 
a  meteorite,  an  aerolite,  a  bolide,  or  half  a 
dozen  other  technical  names.  They  are  really 
all  one  and  the  same  thing — a  meteor. 

The  meteors  are  particles  of  matter  of  various 
sizes  and  compositions,  revolving  through  the 
Solar  System  in  regular  orbits,  usually  very 
elliptical.  Most  of  them  travel  in  swarms,  and 
it  is  when  the  Earth  meets  such  that  we  have 
the  "shower  of  shooting  stars."  The  orbits  of 
most  of  these  swarms  are  well-known. 

The  visibility  of  a  meteor  depends  entirely 
upon  its  collision  with  the  atmosphere  of  the 
100 


Meteors  or  "Shooting  Stars"    101 

Earth  at  very  high  speed — up  to  40  miles  a 
second.  This,  by  the  great  friction,  generates 
a  terrific  heat,  and  the  meteor  (i)  is  either  dis- 
sipated in  fine  dust,  (which  is  often  found  on  the 
Arctic  ice)  or,  (2)  if  it  be  a  large  one,  reaches  the 
surface  of  the  Earth  as  a  much  pitted  and  scarred 
mass  of  stone  or  metal,  or,  (3)  only  grazing  the 
atmosphere,  passes  out  before  being  entirely 
consumed. 

The  Earth  is  undergoing  a  constant  meteoric 
bombardment,  and  were  it  not  for  our  protec- 
tive atmospheric  shield,  we  undoubtedly  should 
suffer  greatly. 

It  is  from  the  visibility  of  meteors  that  at- 
tempts have  been  made  to  gauge  the  height  or 
depth  of  the  atmosphere.  It  would  seem  to  be 
dense  enough  at  about  100  miles  to  cause  the 
friction  necessary  to  produce  the  white  heat  of 
meteors,  at  which  height,  as  a  rule,  they  become 
visible. 

More  meteors  are  seen  in  the  early  morn- 
ing hours  than  in  the  evening  because  at  the 
former  time  we  are  at  the  "bow"  of  the 
Earth,  in  relation  to  its  orbital  motion,  and 
are  in  a  position  to  see  more  "collisions"  than 
in  the  evening,  when  we  are  at  the  "stern," 


102    THe  Essence  of  Astronomy 

and  see  only  those  meteors  which  collide  in 
overtaking  us. 

In  a  meteoric  shower  if  all  the  trails  are  traced 
back,  it  will  be  found  that  they  all  very  closely 
approach  crossing  at  a  definite  point  in  the  sky, 
like  the  ribs  of  an  open  umbrella  viewed  from 
underneath.  This  is  called  the  radiant  point  and 
is  the  direction  from  which  the  meteor  swarm 
is  coming.  The  meteors  of  swarms  all  move  in 
parallel  paths,  and  it  is  perspective  alone  which 
makes  them  appear  to  shoot  in  various  directions. 
If  one  came  from  the  radiant  point  directly 
toward  the  observer,  it  would  appear  as  a  sudden 
brilliant  star  without  motion ,  simply  growing 
more  brilliant  until  it  burned  out,  or  fell  to 
Earth  at  the  observer's  position. 

The  radiant  point  of  a  meteoric  shower  bears 
an  apparent  relative  position  to  the  "fixed  "  stars, 
and  the  swarms  are  known  by  a  name  connected 
with  the  constellation  from  which  they  appear 
to  come:  the  Leonids,  being  apparently  located 
in  the  constellation  Leo ;  the  Perseids  in  Perseus ; 
the  Andromedes  in  Andromeda,  etc. 

Observations  of  great  meteor  showers  run 
back  for  hundreds  of  years,  but  it  was  not  until 
comparatively  recently  (1833)  that  these  showers 


Meteors  or  "SHooting  Stars"     103 

were  known  to  be  periodic.  In  1862  a  remark- 
able connection  was  pointed  out  between  the 
August  meteors,  the  Perseids,  and  the  comet  of 
1862.  Soon,  other  instances  of  like  association 
were  discovered,  and  we  now  know  of  at  least 
eight  different  meteor  swarms  connected  with 
comets.  It  would  seem  that  a  comet  is  but  a 
condensed  section  of  a  meteor  swarm,  or  the 
meteor  swarm  a  disintegrated  comet.  As  an 
example  of  the  latter,  the  case  of  Biela's  comet 
and  the  subsequent  meteoric  shower  is  cited  in 
the  previous  chapter. 

In  size,  meteors  range  from  tiny  particles  up 
to  masses  several  tons  in  weight,  the  largest 
which  has  been  found  on  the  Earth  weighing  five 
and  a  half  tons.  This  fell  in  Brazil  in  1816. 
The  small  ones,  of  course,  never  reach  the  ground, 
and  indeed  the  incandescence  of  the  smallest 
certainly  cannot  produce  light  enough  to  make 
them  visible. 

The  composition  of  such  as  have  reached  us 
for  examination  seems  to  place  them  in  two  main 
classes,  those  of  stone  and  those  of  iron. 

From  the  occluded,  or  absorbed,  gases  that 
may  be  extracted  from  them  in  a  laboratory  by 
heating,  it  is  believed  that  they  must  have  been 


IO4    TKe  Essence  of  Astronomy 

at  one  time  part  of  some  vast  body,  later  torn 
to  pieces,  as  such  gases  could  have  been  ab- 
sorbed only  under  great  heat  and  pressure. 
The  irregular  shape  of  meteorites  also  points 
to  their  being  fragments  of  a  whole,  not  small 
whole  units  in  themselves.  From  these  facts, 
much  has  been  deduced  about  the  origin  of 
our  whole  Solar  System  from  a  great  parent 
body,  shattered  by  some  tremendous  collis- 
ion in  the  ages  past,  meteors  and  comets 
being  the  unswept-up  chips  in  the  great  work- 
shop. 

The  better-known  meteor  swarms  are:  The 
Perseids  of  August,  seen  best  as  a  rule  on  the  loth 
or  nth  of  the  month.  This  is  a  broad  stream, 
evenly  scattered  all  around  its  orbit,  for  we  meet 
them  every  year. 

The  Leonids  of  November,  a  compact  swarm, 
which  the  Earth  meets  on  November  I4th  in  the 
early  morning.  There  was  a  magnificent  dis- 
play of  these  in  1833,  and  again  in  1866,  their 
periodic  time  being  33  years.  They  were  due 
again  in  1899  but  failed  to  appear,  save  as  a  very 
much  inferior  spectacle.  It  was  later  calculated 
that  they  had  passed  close  to  one  of  the  great 
planets  and  their  orbit  in  consequence  had  been 


THE    MILKY   WAY   ABOUT   CHI    CYGNI 

A  splendid  example  of  a  Star  Cloud  is  shown  at 

the  center  of  the  picture 
From  a  photograph  taken  at  the  Lick  Observatory 


SOUTHERN  REGION  OF  ORION 

The  lager  light  blur  in  the  center  of  the  photograph  denotes  the  position 

of  the  great  nebula 
The  bright  streak  is  a  meteor  trail.     Note  the  increase  in  brightness  and 

the  subsequent  fading 
From  a  photograph  taken  at  the  Yerkes  Observatory 


Meteors  or  "  SHooting'  Stars"     105 

changed,  so  that  the  Earth  no  longer  meets  the 
thickest  part  of  the  swarm. 

The  Andromedes  of  November,  a  swarm  seen 
on  the  24th.  This  is  the  one  connected  with  the 
disintegrated  Biela's  comet. 

The  Geminids  seen  about  Dec.  7th. 

The  Orionids  seen  as  a  rule  near  Oct.  iQth; 
and  several  others,  the  dates  of  which  can  be 
found  in  observers'  handbooks. 

All  of  these  swarms  are  more  or  less  scattered 
throughout  their  orbits,  so  that  the  Earth  upon 
the  date  of  crossing  these  orbits,  always  encoun- 
ters a  number  of  them.  The  great  clusters,  how- 
ever, come  only  once  in  their  periodic  times  of 
revolution  about  the  Sun. 


CHAPTER  XV 

ECLIPSES 

THAT  an  eclipse  of  the  Sun  is  caused  by  the 
Moon  passing  between  it  and  the  Earth,  and  that 
an  eclipse  of  the  Moon  is  caused  by  the  latter 
entering  the  great  shadow  the  Earth  casts  into 
space,  is  too  well  known  to  require  any  space 
here. 

As  explained  before,  the  Moon's  orbit  around 
the  Earth  is  inclined,  or  tilted,  to  the  plane  of  the 
ecliptic,  i.  e.,  the  plane  of  the  Earth's  orbit 
around  the  Sun.  It  is  only  when  the  Moon  is 
at,  or  near,  one  of  the  points  where  its  orbit 
cuts  the  plane  of  the  ecliptic,  and,  simultaneously, 
the  Sun,  also,  is  apparently  at,  or  sufficiently 
close  to,  one  of  them  that  an  eclipse  can  occur. 
It  is  for  this  reason  that  the  plane  of  the  Earth's 
orbit  is  call  the  ecliptic.  In  one  year  there  may 
be  as  many  as  seven  eclipses,  five  of  the  Sun  and 
two  of  the  Moon,  or  four  of  the  Sun  and  three  of 
the  Moon;  or  as  few  as  two  eclipses  but  never 
106 


Eclipses  107 

less.  When  there  are  but  two  eclipses  these 
both  must  be  of  the  Sun.  The  Moon  is  never 
eclipsed  more  than  three  times  in  a  year. 

There  is  a  certain  time  cycle,  of  about  19  years, 
called  the  Saros,  at  the  end  of  which  begins  a 
repetition  of  eclipses  similar  to  those  of  the 
previous  19  years.  This  Saros  was  known  to  the 
Chaldeans.  It  is  not  exact,  but  is  close  enough 
to  be  of  practical  value. 

The  reasons  for  the  above  are  all  technical 
and  mathematical,  and,  as  such,  have  no  place 
here. 

Eclipses  of  the  Sun  are  more  numerous  than 
those  of  the  Moon  in  a  ratio  of  about  three  to 
two,  but,  from  any  given  point  on  the  Earth  more 
eclipses  of  the  Moon  may  be  seen  than  those  of 
the  Sun.  A  solar  eclipse,  because  of  the  small 
width  of  the  Moon's  shadow,  can  be  seen  from 
only  a  limited  position  of  the  Earth,  but  an 
eclipse  of  the  Moon  can  be  seen  from  a  whole 
hemisphere  at  the  same  time. 

Eclipses  may  be  roughly  divided  into  two  main 
classes,  total  and  partial. 

ECLIPSES  OF  THE   SUN 

A  total  eclipse  of  the  Sun  is  visible  from  some 


io8    TKe  Essence  of  -Astronomy 

point  on  the  Earth  about  once  every  year  and  a 
half.  At  any  given  place,  however,  one  is  seen 
only  about  once  every  360  years.  This  is  be- 
cause of  the  narrowness  of  the  Moon's  shadow, 
this  averaging  only  about  sixty  to  seventy  miles 
in  width. 

An  eclipse  is  called  total  when  the  Moon's 
disk  entirely  covers  that  of  the  Sun.  Because  of 
the  varying  distances  of  the  Moon  from  the 
Earth,  and  the  Earth,  and  consequently  the 
Moon,  too,  from  the  Sun,  the  apparent  sizes 
of  the  Moon  and  the  Sun  vary.  There  are, 
therefore,  eclipses  where  the  solar  disk  is  barely 
covered,  and  others  where  it  is  completely 
screened.  The  total  stage  of  solar  eclipse  can 
last,  at  most,  only  about  eight  minutes.  The 
shadow  of  the  Moon  travels  at  the  Earth's  sur- 
face with  the  speed  of  a  cannon  ball,  and  only 
under  the  best  conditions  can  an  eight-minute 
totality  occur.  This  can  happen  only  at  the 
Equator,  and  at  the  very  center  of  the  path  of 
1 68  miles  in  width  made  by  the  Moon's  shadow 
when  broadest. 

It  is  only  during  the  total  stage  that  the 
wonderful  Solar  Corona  is  seen.  Whenever  a 
total  eclipse  occurs  it  may  be  seen  also  as  a  partial 


Eclipses  109 

eclipse  on  a  strip  of  the  Earth's  surface  several 
hundred  miles  on  either  side  of  the  path  of 
totality. 

An  eclipse  of  the  Sun  where  the  disk  is  not 
covered  entirely  although  the  Moon  is  passing 
directly  between  the  observer  and  the  Sun,  or 
centrally  as  it  is  termed,  is  called  an  annular 
eclipse  because  of  the  ring  (Latin,  annulus)  of 
the  Sun's  rim  left  shining  around  the  black  disk 
of  the  Moon.  An  annular  eclipse  occurs  when 
the  Moon  is  farthest  from  the  Earth,  and  the 
Earth  and  Moon  are  nearest  the  Sun,  and  in 
consequence,  the  shadow  of  the  Moon  is  not 
long  enough  to  reach  the  Earth.  The  annular 
eclipse  is,  of  course,  only  a  special  form  of 
partial  eclipse. 

The  third  form  of  the  solar  eclipse  is  the 
partial  type.  This  occurs  when  the  Moon  passes 
over  the  Sun's  disk  not  centrally.  It  may  range 
from  an  almost  total  eclipse  to  a  bare  shading  of 
the  Sun's  rim. 

The  total  eclipses  are  those  of  greatest  astro- 
nomical interest. 

ECLIPSES  OF   THE  MOON 

These  are  either   total  or   partial.     A   total 


Iio   TKe  Essence  of  Astronomy 

eclipse  of  the  Moon  occurs  when  it  passes 
entirely  into  the  shadow  of  the  Earth.  The 
diameter  of  the  Earth's  shadow  where  the  Moon 
crosses  it  is  about  5700  miles.  The  Moon, 
therefore,  with  a  diameter  of  2162  miles  can  be 
quite  "out  of  line"  and  still  be  totally  eclipsed. 
When  the  Moon  crosses  the  center  of  the  shadow 
the  total  phase  lasts  about  two  hours.  The 
duration  of  a  non-central  eclipse  varies  accord- 
ing to  what  part  of  the  shadow  is  traversed. 

Owing  to  the  refractive  power  of  the  Earth's 
atmosphere,  the  rays  of  the  Sun  are  somewhat 
bent  around  the  Earth,  so  that  its  shadow  is 
never  clearly  defined,  as  is  that  of  the  Moon; 
and,  even  when  the  latter  is  totally  immersed 
in  the  Earth's  shadow,  its  disk  is  usually  visible, 
glowing  with  a  dull,  copper-colored  light.  The 
color  is  caused  by  the  passage  of  the  Sun's 
rays  through  such  an  extent  of  our  atmosphere. 
From  the  Moon,  during  a  total  lunar  eclipse, 
the  Earth  must  appear  as  a  great  black  disk 
surrounded  by  a  narrow  ring  of  brilliant  light, 
colored  with  sunset  tints.  In  1884,  the  cloudy 
condition  of  the  Earth's  atmosphere  prevented 
the  passage  of  practically  all  this  refracted  light, 
and  the  Moon  for  a  time  was  quite  invisible 


Eclipses  in 

to  the  naked  eye.  This  was  a  most  unusual 
occurrence. 

A  partial  eclipse  of  the  Moon  occurs  when  the 
edge  of  the  Earth's  shadow  only  is  traversed. 
Like  a  partial  eclipse  of  the  Sun,  it  may  range 
from  the  barest  touching  of  the  Moon's  edge  up 
to  an  almost  total  phase. 

Eclipses  were  probably  among  the  very  first 
celestial  phenomena  to  be  studied.  We  have 
records  of  them  dating  back  several  thousand 
years. 

,  The  value  of  a  total  solar  eclipse  lies  in  the 
opportunity  it  gives  for  studying  the  wonderful 
Corona  about  which  so  very  little  is  known. 
Expeditions  are  nearly  always  sent  out  from 
every  large  observatory  to  that  part  of  the  world 
from  which  a  predicted  total  solar  eclipse  will 
best  be  observed.  During  the  totality,  but 
little  attention  is,  nowadays,  paid  to  direct 
observing,  all  efforts  being  centered  upon  obtain- 
ing as  many  good  photographs  of  the  phenomenon 
as  possible.  Direct  observations  can  last  only 
during  the  eclipse,  while  photographs  may  be 
effectively  studied  for  years,  and  form  a  perma- 
nent record  of  that  eclipse.  Dry,  high  countries 


112    TTHe  Essence  of  Astronomy 

are  of  course,  the  most  suitable  for  the  placing 
of  such  eclipse  stations,  since  a  few  moments 
of  cloud  at  the  wrong  time  will  prevent  all  obser- 
vation, and  render  entirely  useless  a  costly  and 
laborious  expedition.  Unfortunately,  it  is  not 
always  possible  to  find  a  good  station  in  the  path 
of  the  eclipse,  and,  in  consequence,  many  an 
expedition  has  gone  for  naught. 

Until  the  spectroscope  proved  its  ability  to 
view  them  under  ordinary  conditions,  the  study 
of  the  Solar  Prominences  was  also  restricted 
to  the  scant  few  moments  of  totality  during  an 
eclipse. 

Lunar  eclipses  present  to  the  scientist  no  fea- 
tures of  special  interest. 

The  ability  to  reckon  backward  for  centuries 
eclipses  of  both  the  Sun  and  the  Moon  has  helped 
greatly  in  establishing  historical  dates  of  events 
occurring  near  eclipses  mentioned  in  old  records. 


CHAPTER  XVI 

THE  FIXED   STARS 

JUST  as  our  Sun  is  a  star,  so  the  stars  are 
suns,  great  flaming  masses  of  gases  at  enormous 
temperatures,  brilliant  with  a  light  of  their  own, 
and  so  huge  that  they  may  continue  to  give 
light  and  heat  for  millions  of  years  before  they 
cool.  "What  has  been  said  of  the  Sun  probably 
applies  in  a  greater  or  less  degree  to  the  great 
majority  of  the  stars, "  but  so  vast  is  the  distance 
which  separates  even  the  nearest  one  from  us 
that,  despite  their  enormous  volume,  they  appear 
only  as  mere  points  of  light.  It  is  safe  to  say, 
however,  that  the  majority  of  them  are  larger 
and  hotter  than  the  Sun  and  like  the  latter  rotate. 
It  is  probable,  this  from  analogy  only,  that  a 
great  many  of  them  also  have  their  family  of 
planets. 

The  composition  of  the  stars  as  well  as  their 
physical  condition  varies.  The  spectroscope 
reaches  across  the  great  void  of  space,  and,  to  a 

8  113 


H4    THe  Essence  of  Astronomy 

large  extent,  analyzes  these  bodies  for  us.  We 
find  that  many  well-known  substances,  such  as 
iron,  hydrogen,  calcium,  etc.,  seem  to  be  uni- 
versally distributed  throughout  the  stars,  but 
we  also  find  that  variety  in  composition  is  not 
unusual.  Some  stars  show,  under  spectroscopic 
analysis,  substances  which  we  do  not  know  on 
Earth,  and  again  seem  to  lack  substances  which 
are  main  factors  in  the  make-up  of  other  stars. 
Indeed,  the  stars  have  been  to  a  certain  extent 
classified  according  to  their  composition.  It  is 
interesting  to  note,  in  connection  with  the  un- 
known elements  of  the  universe,  that  helium  was 
found  in  the  Sun,  hence  its  name,  long  before  it 
was  discovered  upon  the  Earth.  It  is  quite 
possible  therefore  that  chemistry  will  reduce  the 
element  or  elements  causing  the  enigmatical 
lines  in  the  spectrum  of  the  "Orion  type"  stars, 
a  class  which  seems  to  stand  quite  alone. 

It  seems  probable  that  most  of  the  stars  are 
much  less  dense  than  the  Sun;  in  many  cases 
it  is  possible  to  measure  stellar  density  with  a 
certain  amount  of  accuracy  and  in  all  such 
instances  these  densities  are  "far  less  than  any 
of  our  solid  or  liquid  substances;  frequently  no 
greater  than  that  of  air,  sometimes  even  less." 


The  Fixed  Stars  115 

It  is  also  probable  that  most  of  the  stars 
are  vastly  larger,  and  are  thousands  of  times 
more  luminous,  than  the  Sun.  A  star  such  as 
Canopus,  visible  from  southern  latitudes  only, 
is  known  to  be  over  2500  trillions  of  miles 
distant,  and  quite  possibly  much  farther  away, 
and  yet  it  shines  with  a  brilliancy  surpassed  by 
only  one  of  the  nearest  stars. 

The  stars  are  called  "fixed  stars"  because  of 
their  apparent  permanent  relative  positions,  in 
contradistinction  to  the  planets,  the  wanderers, 
which,  to  the  naked  eye,  look  like  stars,  but  can 
be  seen  to  shift  their  positions  from  night  to 
night. 

From  any  point  on  the  Earth,  the  heavens 
appear  as  a  great  dome,  or  one  half  of  a  vast 
sphere,  with  the  observer  directly  beneath  the 
highest  point  of  the  arch.  Were  it  possible  to 
remove  the  Earth,  and  leave  the  observer  sus- 
pended in  space,  it  would  be  seen  that  the  sphere 
was  complete,  with  the  observer's  position  ex- 
actly at  the  center.  This  sphere  seems  to  be 
studded  with  the  stars,  all  of  them  appearing  as 
equally  distant.  This,  of  course  is  but  an 
optical  illusion.  The  Moon  seems  equidistant 
with  the  stars,  as  do  also  the  planets,  and  yet 


Ii6    THe  Essence  of  Astronomy 

Jupiter  is  many  millions  of  miles  away,  the 
Moon  but  half  as  many  thousands,  and  even 
the  nearest  of  the  stars  many  million  millions. 
The  astronomers,  however,  make  good  use  of  this 
illusion  and  treat  this  apparent  sphere  as  if 
it  were  an  existing  concrete  object.  Upon  it 
the  positions  of  the  heavenly  bodies  are  marked 
by  a  system  exactly  corresponding  to  the  geo- 
graphic method  of  denning  the  positions  of  a 
place  by  stating  its  latitude  and  longitude. 

We  do  not  see  the  stars  in  the  daytime  because 
of  the  diffusion  through  our  atmosphere  of  the 
brilliant  light  of  the  Sun.  Were  our  atmosphere 
removed,  the  stars  would  be  always  readily 
visible  at  even  high  noon.  Even  now,  under 
the  present  conditions,  it  is  possible  to  see 
some  of  the  brighter  stars  in  the  daytime  if  the 
observer  be  stationed  at  the  bottom  of  a  suffi- 
ciently deep  shaft,  thus  cutting  off  the  larger 
part  of  the  daylight. 

This  Celestial  Sphere,  as  it  is  called,  appears  to 
revolve  constantly  about  the  Earth  from  east 
to  west,  making  one  revolution  in  a  day.  As 
we  know  now,  this  motion  is  only  apparent,  and 
is  caused  by  the  rotation  of  the  Earth  in  the 
opposite  direction.  For  this  reason,  the  axis 


TKe  Fixed  Stars  117 

of  the  Celestial  Sphere  is  merely  the  axis  of  the 
Earth  prolonged;  its  Poles,  the  points  where  the 
Earth's  axis  cuts  its  surface;  and  its  Equator 
similar  to  that  of  the  Earth,  a  line  encircling  it 
equidistant  from  these  poles. 

The  present  north  pole  of  the  Celestial  Sphere 
is  close  to  a  bright  star,  which  is  called  in  con- 
sequence Polaris,  or  the  North  Star,  but,  owing 
to  one  of  the  Earth's  motions,  a  "wobble"  on  its 
axis  like  a  huge  top,  this  pole  is  not  a  constant 
point.  In  the  course  of  about  26,000  years  the 
Earth's  axis  describes  a  circle  on  the  Celestial 
Sphere,  so  that  13,000  years  from  now  Polaris 
will  be  far  from  being  the  North  Star,  which 
honor  will  be  held  by  the  bright  star  Vega  of  the 
Lyre.  But,  13,000  years  later,  Polaris  once 
again  will  be  in  its  present  relative  position 
to  the  North.  Some  4000  years  ago  Thuban  of 
the  Dragon  was  the  North  Star,  and,  it  is  said, 
it  was  for  the  purpose  of  viewing  this  star  at 
all  times  that  the  great  slanting  tunnel  in  the 
Pyramid  of  Cheops  was  built. 

The  stars  are  not  evenly  divided  about  the 
Celestial  Sphere.  There  is  surrounding  it  a 
great  encircling  band  of  light,  called  the  Milky 
Way,  or  Galaxy,  which,  under  telescopic  observa- 


n8    XHe  Essence  of  Astronomy 

tion,  resolves  into  thousands  upon  thousands  of 
stars,  so  apparently  close  together  as  to  shine 
not  individually  but  as  an  almost  solid  belt 
of  light.  This  is  more  fully  described  in  the 
next  chapter.  In  and  near  this  Galactic  belt  is 
where  most  of  the  stars  seem  gathered,  while  at 
the  points  of  the  heavens  most  distant  from  this 
ring  (the  Galactic  Poles)  the  stars  are  compara- 
tively few  and  far  between.  It  is  this  appear- 
ance which  led  to  a  theory  that  the  Universe 
is  in  the  shape  of  a  great  disk  with  the  Solar 
System  about  at  the  center,  so  that  looking 
"edgewise"  or  through  its  broad  dimensions 
there  are  far  more  stars  to  see  than  when  look- 
ing flatwise,  or  through  the  thin  dimension. 
The  theory  that  the  Galaxy  is  an  actual  belt 
surrounding  the  sphere,  is  the  more  generally 
accepted  of  the  two.  However,  this,  at  present, 
can  be  really  only  a  matter  of  conjecture.  Of 
the  shape  or  extent  of  the  Universe  we  have  no 
definite  knowledge. 

Aside  from  the  distribution  of  the  stars  just 
spoken  of,  the  apparent  grouping  of  them  into 
"constellations"  has  been  noticed  from  the 
most  ancient  times,  and  names  given  to  such 


LARGER    STARS    OF   THE    PLEIADES 

Exposure  too  short  to  photograph  nebulosity  shown  in  frontispiece.     Note  the  apparent 
distortion  of  largest  star.      This  shows  as  four  stars  in  even 

a  small  telescope 
From  a  photograph  taken  at  the  Yerkes  Observatory 


TKe  Fixed  Stars  119 

groups.  The  names  which  are  now  used  come  to 
us  from  the  time  of  Ptolemy  in  the  second 
century,  and  are,  for  the  most  part,  those  of  the 
mythological  Greek  heroes  and  their  fellows, 
whose  forms  the  ancients  saw  translated  to  the 
skies.  These  constellations  are  merely  appar- 
ent. Two  stars  seemingly  side  by  side  may  be 
actually  billions  of  miles  apart,  one  behind  the 
other,  so  to  speak,  from  the  direction  we  view 
them.  The  stars  forming  the  constellations 
have  now  been  lettered  and  numbered,  and  are 
referred  to  as  Alpha  Canis  Majoris,  A  of  the 
Great  Dog;  Omicron  Ceti,  o  of  the  Whale,  etc. 
Many  of  the  brighter  stars  also  have  individual 
names,  the  above  two  being  Sirius  and  Mira, 
respectively. 

Most  of  the  constellations  bear  little  resem- 
blance to  the  fancied  figures  assigned  to  them  by 
the  ancients,  but  habit  is  strong,  and,  probably 
for  many  years,  they  will  bear  their  present 
names.  It  is  not  possible  to  describe  these 
groupings  properly  without  devoting  more  space 
than  is  available  here,  and  without  assisting 
charts.  There  are  several  books  listed  in  the 
bibliography  written  and  illustrated  with  the 
proper  maps  for  just  this  purpose. 


I2O    THe  Essence  of  Astronomy 

The  number  of  the  stars  is  not  known  with  any 
approach  to  exactness.  It  is  certainly  very 
great.  Estimates  have  been  given  up  to  well 
over  100,000,000. 

The  number  visible  to  the  naked  eye  is  about 
6000.  Only  about  one  half  of  these,  of  course, 
are  above  the  horizon  of  a  place  at  any  one  time, 
and,  owing  to  the  absorption  of  light  by  our  at- 
mosphere, those  close  to  the  horizon  are  not 
visible.  That  our  air  does  absorb  light  is 
demonstrated  by  the  ease  with  which  one  may 
gaze  at  the  Sun  just  as  it  is  setting,  while  its 
noonday  brilliance  is  unbearable.  At  one  time 
and  place  therefore,  even  under  the  most  per- 
fect conditions,  only  about  2000  stars  may  be 
seen  without  telescopic  aid. 

With  every  increase  in  the  size  of  telescopes 
more  stars  appear,  and  the  photographic  plate 
with  its  power  of  retaining  the  cumulative 
effects  of  light  during  a  long  exposure  show  stars 
which  it  is  improbable  will  ever  be  seen  with 
any  telescopes  constructed  like  the  present  ones. 

The  stars  are  apparently  different  in  size  and 
brilliancy  and  this  has  led  to  their  classifications 
by  magnitudes.  The  magnitude  of  a  star  tells 


The  Fixed  Stars  121 

nothing  about  its  actual  size,  but  only  of  its 
brilliancy  as  seen  from  the  Earth.  A  compara- 
tively small  star  in  some  cases  far  outshines  one 
which  we  know  must  be  of  stupendous  dimen- 
sions, but  which  is  much  farther  away. 

There  are  about  20  stars  of  the  first  magnitude, 
60  of  the  second  magnitude,  130  of  the  third 
magnitude,  each  star  of  one  magnitude  being 
somewhat  between  two  and  two  and  one  half  times 
more  brilliant  than  one  of  a  magnitude  lower. 
The  photographic  plate  has  recorded  stars  of  the 
2Oth  magnitude,  giving  to  us  thousands  of  times 
less  light  than  the  first  magnitude  stars.  These 
magnitudes  merge  into  each  other  on  a  decimal 
scale,  and  a  star  may  be  of  magnitudes  9.2  or 
3.4  etc. 

The  distances  of  the  stars  are  almost  incon- 
ceivable. Beyond  the  limits  of  the  Solar  System, 
whose  dimensions  we  have  seen  are  in  themselves 
huge,  stretches  empty  space  for  many  million 
times  a  million  miles.  Our  unit  of  a  mile  is 
useless  for  such  vast  measurements,  and  even 
the  distance  of  the  Earth  from  the  Sun,  the 
"yardstick"  which  is  the  standard  for  the  Solar 
System,  is  too  small.  Astronomers  have  been 


122    TTHe  Essence  of  Astronomy 

forced  to  find  a  unit  far  greater  than  this: 
namely,  the  "light-year,"  which  is  the  distance 
covered  by  light  traveling  for  a  year  at  the  amaz- 
ing speed  of  186,000  miles  per  second.  The 
light-year  is  equivalent,  therefore,  to  5,873,- 
286,000,000  miles.  Some  of  the  stars  in  the 
Milky  Way  must  be  thousands  of  these  light- 
years  distant. 

The  nearest  star  neighbor  to  our  Sun  is  the 
third  brightest  star  of  the  heavens,  Alpha  of  the 
Centaur,  a  southern  constellation,  and  its  dis- 
tance is  4.3  light-years,  about  25  trillions  of  miles ; 
the  next  nearest  is  a  small  telescopic  star  in  the 
northern  heavens  bearing  no  name  but  merely 
a  catalogue  number,  which  is  8.1  light-years 
distant.  Sirius,  the  Dog  Star,  the  most  brilliant 
of  the  heavenly  host,  is  8.7  light-years  away. 
These  distances  are  all  known  with  fair  accuracy. 

But  a  very  few  of  the  stars  are  near  enough 
to  be  measured,  about  one  hundred  only  out  of 
the  many  millions.  In  some  cases  it  has  not  been 
possible  to  ascertain  the  real  distance  of  the 
star,  but  it  has  been  demonstrated  certainly 
that  the  star  can  not  be  nearer  than  a  definite 
limit.  This  applies  to  several  of  the  bright 
stars  such  as  Canopus,  previously  cited ;  Rigel, 


TKe  Fixed  Stars  123 

the  gem  of  Orion;  Deneb,  A  of  the  Swan;  and 
Spica,  A  of  the  Virgin,  which  all  must  be  over 
500  light-years  away.  Their  vast  size  and  great 
light-giving  power  may  therefore  be  imagined, 
when  from  such  distances  they  can  not  only 
rival  in  brilliancy  but  actually  outshine  stars 
hundreds  of  light-years  nearer,  which  latter 
nevertheless  are  known  to  be  far  greater  than 
our  Sun. 

As  was  said,  the  distances  of  some  of  the  stars 
in  the  Milky  Way  are  placed  at  thousands  of 
light-years. 

One  of  the  effects  of  the  great  star  distances 
is  that  we  do  not  see  the  heavens  as  they  are, 
but  as  they  were.  The  light  which  is  just  reach- 
ing us  from  the  nearest  star  started  about  four 
years  and  four  months  ago,  so  that  we  are  now 
seeing  that  star  where,  and  as  it  was  when  the 
light  left  it.  Were  some  distant  star  suddenly 
extinguished  to-day,  we  should  continue  to  see 
it  shine  night  after  night  for  even  hundreds  of 
years. 

Not  even  the  most  casual  observer  of  the  stars 
can  fail  long  to  note  the  variance  in  color  between 
many  of  them.  Some,  such  as  Vega  of  the  Lyre, 


124    THe  Essence  of  Astronomy 

glitter  with  the  blue-white  of  a  first-water  dia- 
mond; others,  like  Arcturus  of  the  Herdsman, 
shine  with  a  deep  yellow  light;  and  still  others, 
like  Ant  ares  of  the  Scorpion,  Aldebaran  of  the 
Bull,  and  Betelgeuse  of  Orion,  glow  red  against 
the  sky.  Many  telescopic  stars  have  even  more 
pronounced  colors,  some  hanging  like  drops  of 
blood  in  the  heavens  and  others  glittering  like 
wonderful  sapphires. 

These  colors  arise  from  different  physical  con- 
ditions of  the  stars,  and,  to  a  certain  extent,  from 
their  composition.  According  to  the  accepted 
theory  at  present,  the  stars  of  the  brilliant  blue- 
white  are  the  younger,  while  those  of  the  deep 
yellow  and  reddish  colors  are  the  older,  the  red 
stage  showing  the  greater  stellar  age.  Our 
Sun  is  a  yellowish  star  and  is  supposedly  show- 
ing signs  of  advancing  years. 

Far  from  being  fixed,  it  is  known  that  all  the 
stars  are  in  motion;  again  it  is  only  their  vast 
distances  which  deceive  us.  A  very  moderate 
stellar  rate  is  ten  to  fifteen  miles  per  second, 
864,000  to  1,000,000  miles  per  day,  and  yet  it 
requires  the  most  careful  and  minute  measure- 
ments to  show  any  change  in  a  star's  position 


The  Fixed  Stars  125 

even  after  many  years.  Despite  this  rushing 
helter-skelter  at  these  speeds  and  vastly  greater 
ones,  the  stars  have  displaced  themselves  so 
little  that,  did  any  of  the  ancient  Grecian  astrono- 
mers view  the  heavens  to-day,  he  would  find 
the  constellations  wheeling  through  the  night 
almost  as  he  left  them  twenty-five  hundred  years 
ago.  Eventually,  however,  the  aspect  of  the 
heavens  will  change  and  the  constellations  lose 
all  resemblance  to  their  present  forms. 

The  most  remarkable  body,  from  the  point 
of  speed,  is  a  telescopic  star  known  only  by  a 
catalogue  number,  Groombridge  1830.  This  is 
hurtling  through  space  at  the  terrific  speed  of 
over  200  miles  per  second.  It  is  called  the 
"runaway  star, "  and  no  explanation  for  its  speed 
is  yet  at  hand.  The  mighty  Arcturus,  at  least 
100  times  our  Sun  in  volume,  is  rushing  along  at 
the  speed  of  nearly  90  miles  per  second;  61 
of  Cygnus,  interesting  as  the  first  star  whose 
distance  was  satisfactorily  measured,  speeds  at 
50  miles  per  second,  and  many  are  known  to 
have  velocities  of  from  twenty  to  one  hundred 
miles  per  second.  It  is  possibly  safe  to  say  that 
an  average  star  speed  is  about  fourteen  to 
twenty  miles  per  second. 


126    THe  Essence  of  .Astronomy 

Besides  the  motions  of  the  stars  across  our  line 
of  sight,  which  results  in  changes  of  their  posi- 
tions on  the  Celestial  Sphere,  they  have  radial 
velocity,  motion  toward  or  away  from  us,  directly 
in  our  line  of  sight.  These  motions  are  meas- 
ured by  the  spectroscope,  which  shows  us  the 
speed  at  which  a  star  is  approaching  the  Earth. 
When  the  proper  corrections  are  made  for  the 
motion  of  the  Sun,  which  as  has  been  stated  is 
rushing  along  with  the  planets  at  a  rate  of  about 
twelve  miles  a  second,  and  for  the  motions  of  the 
Earth,  as  it  revolves  about  the  Sun,  it  is  possible 
to  give  with  a  really  marvelous  accuracy  the 
star's  radial  velocity.  It  is  by  reckoning,  from 
the  displacement  of  the  star  on  the  celestial 
sphere,  its  radial  velocity  and  its  distance  from 
us,  that  the  true  motion  of  the  star  in  space  is 
calculated. 

There  have  been  attempts  to  show  that  the 
stars  were  moving  in  great  orbits  about  some  Cen- 
tral Sun,  and  for  a  time  it  was  claimed  that  this 
central  body  was  the  brightest  star  of  the  Pleiades 
group.  This  now  has  been  absolutely  disproved, 
and  no  trace  of  such  orbital  motions  has  been 
found.  There  has  been  shown,  however,  a  great 
star  drift  or  rather  two  drifts,  as  if  two  universes 


THe  Fixed  Stars  127 

had  met  and  were  passing  through  one  another. 
These  drifts  are  now  being  studied  most  care- 
fully. 

Many  people  ask  why  a  star  "twinkles.  ' 
This  is  entirely  caused  by  the  refraction  and 
disturbances  in  the  Earth's  atmosphere.  Were 
this  removed,  the  stars  would  blaze  steadily  in  a 
deep  black  sky  with  a  glory  never  to  be  seen  from 
the  Earth's  surface.  The  planets  do  not  twinkle, 
as  the  beams  of  light  coming  from  them  are 
comparatively  "thick"  and  sturdy,  while  such 
are  the  distances  of  the  stars  that  but  the  tiniest 
thread  of  their  light  reaches  us;  so  tiny  is  this 
that  the  slightest  disturbance  of  the  air  causes 
enough  refraction  to  make  the  light  flicker.  A 
planet  close  to  the  horizon,  when  its  light  must 
penetrate  more  and  denser  air,  most  of  which  is 
in  a  greatly  disturbed  condition,  will  be  seen  to 
' '  flicker ' '  as  does  a  star.  Mercury  indeed ,  which 
is  never  seen  far  above  the  horizon,  is,  as  pre- 
viously stated,  sometimes  called  the  "  Twinkler. " 


CHAPTER   XVII 

NEBULA 

IN  the  star-catalogs  of  the  early  writers  we 
find  mentioned  a  class  of  "nebulous  or  cloudy 
stars. "  The  telescope  proved  that  the  very  great 
majority  of  these  are  merely  clusters  of  stars  so 
apparently  close  together  that  they  shine,  except 
under  fairly  high  magnification,  as  a  blur  of  misty 
light.  With  the  improvement  of  the  telescope, 
however,  many  other  "patches  of  light"  were 
found,  some  of  which  could  be  resolved  into 
separate  stars,  while  others  could  not.  The 
former  of  these  were  called  star-clusters,  and  the 
latter  nebula. 

There  are  many  thousands  of  these  nebulae; 
but  only  about  ten  thousand  of  them  have  been 
cataloged,  and  only  two  are  visible  to  the  naked 
eye,  the  great  nebula  in  Andromeda,  and  the 
great  Orion  nebula. 

For  many  years,  the  nebulas  were  considered 
as  anomalies  in  the  cosmic  system.  Now, 
however,  they  are  believed  to  be  a  regular  and 
128 


Nebulae  129 

usual  step  in  stellar  evolution.  It  is  considered 
that  they  are  stellar  systems  in  embryo.  Their 
distribution  in  space  would  seem  to  support 
this,  being  quite  the  opposite  of  star  distri- 
bution. That  is,  where  the  stars  are  the  most 
numerous,  in  the  Milky  Way  and  the  neighbor- 
ing regions,  the  nebulae  are  scarce;  and,  con- 
versely, at  the  poles  of  the  Galaxy,  where  the 
stars  are  few  and  far  between,  the  nebulae  are  to 
be  found  in  the  greatest  numbers.  There 
seems  nothing  improbable,  therefore,  in  the 
theories  that,  in  the  Galaxy,  the  stars  have,  so 
to  speak,  consumed  in  their  formation  the 
material  of  which  the  nebulae  consist;  or,  on  the 
other  hand,  as  some  astronomers  have  sug- 
gested, the  nebulae  have  been  formed  by  the 
disintegration  of  stars.  The  former  theory  seems 
to  be  more  generally  accepted,  but  whichever 
view  is  correct,  the  nebulae  are  certainly  a  regu- 
lar part,  at  one  extreme  or  the  other,  of  stellar 
evolution.  It  is  quite  possible  that  both  views 
are  right,  and  that  from  the  disintegration  of  one 
system  springs  a  new  one. 

In  considering  the  evolution  of  stars  from 
nebulae,  or  vice  versa,  it  is  well  to  remember 
that  in  many  cases  the  two  types  are  unques- 

9 


130    THe  Essence  of  Astronomy 

tionably  in  close  connection.  The  well-known 
group,  the  Pleiades,  is  suffused  throughout  by  a 
faint  nebulous  light  (see  frontispiece),  and  each 
of  its  stars  is  enveloped  in  a  dense  fog  of  light- 
mist,  to  such  an  extent  that  there  can  be  no 
doubt  as  to  either  the  origin  of  the  stars  from 
a  great  nebula,  or  the  beginning  of  a  nebula  from 
the  emanations  of  the  stars. 

There  are,  also,  isolated  nebulous  stars  to 
support  the  evidence  which  points  to  an  intimate 
relation  between  these  two  apparently  different 
forms  of  matter.  "There  is  a  vast  difference 
between  a  nebulous  star,  and  a  star  in  a  nebula" 
(Swift).  For,  while  a  star  in  a  nebula  may  only 
appear  to  be  there,  as  a  result  of  perspective, 
the  luminous  halo  surrounding  a  nebulous  star 
cannot  be  disassociated  from  the  star  itself.  A 
nebulous  star  has,  as  a  rule,  the  appearance  of  a 
rather  bright  point  of  light  at  the  center  of  an 
aureole  of  faint  luminosity  which  fades  off  into 
the  darkness  of  space  in  an  almost  perfect 
gradation. 

Practically  all  nebulous  stars  show,  under  the 
analysis  of  the  spectroscope,  characteristics 
which  are  associated  with  "youthful"  stars,  and 
this  also  would  seem  to  lend  support  to  the 


Nebulae  131 

theory  that  stars  are  actually  formed  from 
condensations  in  nebulae,  and  that  a  nebulous 
star  is  one  taking  the  early  steps  along  its  path 
of  evolution. 

Nebulas  may  be  classified,  in  regard  to  shape, 
as:  Spiral;  Ring,  or  Annular;  Planetary;  and 
Irregular. 

The  SPIRAL  form  of  nebula  is  by  far  the  most 
common.  It  is  to  this  class  that  the  great 
Andromeda  nebula  belongs.  One  of  the  best 
examples  of  this  shape  and  class,  the  "Whirl- 
pool" nebula  in  the  constellation  Canes 
Venatici,  is  shown  in  the  accompanying  illus- 
tration. The  resemblance  to  a  fireworks 
"  pin-wheel "  is  very  striking,  and  it  is,  in- 
deed, most  probable  that  just  such  a  rapid 
whirling  motion  is  there  present.  That  we 
cannot  see,  and  have  as  yet  been  unable  to  meas- 
ure, any  consequent  change  in  form  or  position, 
is  due  both  to  the  immense  sizes  of  the  nebulae, 
and  to  the  almost  inconceivable  distances  which 
stretch  between  them  and  the  Earth.  Each  of 
these  spiral  nebulae  shows  a  marked  large  central 
condensation,  while  along  the  curved  arms  that 
whirl  from  this  centre  are  beaded  smaller  knots 


132   THe  Essence  of  Astronomy 

and  thickenings,  so  that  it  is  very  easy  to  deduce 
their  ultimate  condition  as  that  of  a  vast  system, 
where  perhaps  many  hundreds  of  smaller  stars, 
which,  of  course,  may  cool  into  planets,  swing 
in  great  orbits  about  a  huge  central  sun. 

The  RING,  or  ANNULAR,  NEBULA  may 
easily  be  but  a  somewhat  unusual  form  of  the 
spiral, — a  "  drawn-out "  spiral  like  a  spring, 
which,  when  viewed  from  "on  top"  appears 
simply  as  a  ring.  One  of  the  best  known  ring 
nebulae  has  recently  been  proved  to  be  of  this 
helical  shape.  The  ring  nebulas  are  always  darker 
at  the  centers  than  at  the  borders.  The  most 
famous  one,  in  the  constellation  Lyra,  appears  in 
a  large  telescope  as  a  well  marked  ring  of  light 
surrounding  a  dark  central  space,  in  the  middle 
of  which,  or  through  the  middle  of  which ,  from 
possibly  far  beyond,  shines  a  small  star. 

The  PLANETARY  NEBULA  were  so  named  by 
the  elder  Herschel,  because  of  their  appearance. 
They  are  small  (comparatively  speaking),  cir- 
cular or  elliptical  nebulae,  which  through  a 
telescope  present  an  almost  evenly  illuminated 
disk,  somewhat  like  that  of  a  planet.  Nearly 
all  of  them  may  be  recognized  by  a  characteristic 
greenish  tinge. 


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Nebulae  133 

The  IRREGULAR  NEBULAE  are  of  almost  any 
shape.  The  great  nebula  of  Orion,  shown  in 
the  illustration,  facing  page  136  is  the  most 
famous  of  this  class.  Some  of  these  are  but 
the  faintest,  misty  wisps  of  cloudy  light,  de- 
tected only  by  the  photographic  plate,  while 
others  are  comparatively  bright,  and  are  easily 
observed  in  even  a  small  telescope.  A  number 
of  the  irregular  nebulae  have  special  and  distinc- 
tive names,  such  as :  The  Dumb-bell  Nebula,  The 
Crab  Nebula,  The  North  America  Nebula,  The 
Owl  Nebula.  These  names  have  been  given 
to  them  because  of  a  very  real,  not  a  fancied, 
resemblance  to  their  namesakes. 

The  real  constitution  of  nebulae  is  yet  largely  a 
matter  of  speculation.  The  spectroscope  divides 
them  into  two  classes,  the  white  and  the  green. 

The  white  nebulae,  to  which  class  the  spirals 
belong,  and  which  are  in  consequence  by  far 
the  more  numerous,  show,  under  the  spectro- 
scope, that  they  probably  are  composed  of  solid 
particles;  but,  as  gases  under  pressure  would 
give  a  similar  spectrum,  this  is  not  yet  certain. 
There  is  such  difficulty  in  collecting  enough  light 
from  a  nebula  to  obtain  a  good  spectrum  of  it, 


134   THe  Essence  of  Astronomy 

that  great  advances  in  the  quality  of  our  instru- 
ments must  be  made  before  a  definite  knowledge 
of  this  sort  can  be  acquired. 

It  is  known  definitely,  however,  that  the 
irregular  and  the  planetary  nebulae,  as  well  as 
other  subdivisions  of  the  green  nebulae  which  have 
not  been  mentioned,  are  of  a  gaseous  composition. 
Their  spectra  are  too  evident  to  allow  uncer- 
tainty. These  gaseous  nebulae,  the  planetary  in 
particular,  contain  a  substance  yet  to  be  found  on 
the  Earth.  This  has  a  characteristic  marking 
in  the  spectrum  and  is  easily  recognized.  It 
has  been  given  the  name  "nebulium." 

It  has  been  shown  that  all  nebulae  must  be  of 
extreme  rarity,  far  beyond  that  obtainable  by 
artificial  means.  Stars  shine  in  and  through 
them  with  no  appreciable  diminution  of  light. 
Granting  the  truth  of  the  lowest  estimate  for  the 
distance  of  the  Andromeda  nebula,  its  gravita- 
tional pull  on  the  Earth  would  be  as  great  as  that 
of  the  Sun  had  it  a  density  equal  to  300.000.006 
that  of  our  "day-star."  AS  we  can  find  in 
the  Solar  System  no  disturbing  effect  which  can 
be  attributed  to  such  an  exterior  gravitational 
pull,  it  is  evident  that  this  particular  nebula,  at 


Nebulae  135 

least,  must  be  of  a  density  approaching  "nothing 
at  all." 

The  sizes  of  most  of  the  nebulae  are  probably 
vast  beyond  belief.  It  has  been  demonstrated, 
with  an  approach  to  certainty,  that  the  great 
Andromeda  nebula  cannot  be  less  than  eight 
light-years  in  diameter — that  is,  it  would  stretch 
from  our  Sun  into  space  for  a  distance  nearly 
twice  as  great  as  that  which  intervenes  between 
us  and  the  nearest  star,  and  yet  this  star,  huge 
as  it  is,  shines  to  us  as  only  the  tiniest  point 
of  light. 

All  the  nebulae  lie  at  tremendous  distances. 
Most  of  them  are  beyond  the  limits  of  measure- 
ment; this  again  points  to  the  enormous  areas 
covered  by  them. 

The  nebulae  seem  to  be  in  motion  like  all  other 
bodies  in  the  universe.  Their  proper  motions 
have  not  been  measured,  but  the  spectroscope 
reads  their  radial  velocities,  their  motions  to 
and  from  us. 

It  is  not  at  all  impossible  that  there  exist  many 
dark  nebula,  of  the  same  general  nature  as  those 
shining  with  a  light  of  their  own,  but  flying 


136    THe  Essence  of  Astronomy 

unnoticed  through  space.  In  fact,  the  dark 
nebula  has  been  called  upon  to  explain  several  of 
the  unsolved  problems  of  the  universe,  such  as 
the  temporary  star  and  the  "coal-sacks"  in  the 
Milky  Way.  As,  however,  we  can  study  only 
the  visible  members  of  the  stellar  horde  and  a 
few  of  the  dark  members  by  their  effect  upon 
their  visible  fellows,  the  non-lucent  nebula  must 
remain,  for  the  present  at  least,  merely  specu- 
lation, however  probable  a  one. 

Just  what  makes  a  nebula  luminous  is  also 
unknown.  It  is  impossible  for  matter  in  such 
a  rarefied  form  to  be  what  we  call  hot,  so  that 
they  cannot  be  incandescent,  as  is  possible,  and 
probable,  with  the  stars.  Electrical  energy  is 
the  most  plausible  explanation,  this  produc- 
ing the  luminous  effect  somewhat  as  in  the 
vacuum  tube  in  a  laboratory. 

It  has  been  the  photographic  plate  which  has 
enabled  us  to  study  best  these  "ghosts  of 
matter."  Most  of  them  are  very  disappoint- 
ing sights  in  even  the  great  telescopes,  but  the 
camera,  with  its  ability  to  retain  the  cumulative 
effect  of  light,  brings  out  a  wonderful  wealth  of 
detail  far  beyond  the  eye's  power  to  grasp. 

A  theory  has  been  advanced  that,  in  case  of 


Nebulae  137 

the  Andromeda  nebula  particularly,  we  are  see- 
ing beyond  our  own  universe,  and  viewing 
another  great  and  similar  system  far  out 
in  space.  While  this  would  account  for  the 
"continuous  spectrum"  showing  the  probability 
of  the  nebula  being  composed  of  solids  (and, 
therefore,  says  the  originator  of  the  theory,  why 
not  actual  stars  too  far  away  to  be  seen  sepa- 
rately), other  and  recent  measurements  and 
observations  do  not  support  this  splendid 
effort  of  imagination. 


CHAPTER  XVIII 

FREAKS  AND  ODDITIES  OF  THE  SKIES 

THERE  are  many  curiosities  in  the  skies  to 
which  individual  space  must  be  devoted.  These 
may  be  divided  into  Star  Groupings  (other  than 
the  constellations),  and  the  Abnormal  Stars, 
some  of  the  last  being  true  freaks  indeed. 

STAR  GROUPINGS 

These  may  be  classified  as :  The  Milky  Way  or 
Galaxy,  spoken  of  in  the  last  chapter,  the 
great  grouping;  and  the  small  ones,  the  Star 
Clouds,  Star  Swarms,  Star  Clusters,  and  Star 
Streams.  There  must  also  be  mentioned  those 
strange  "holes"  in  the  universe  known  as  the 
"Coal-Sacks." 

THE  MILKY  WAY.     As  described  before,  this 
seems  to  be — and  a  certainty  in  the  matter  is  be- 
ing  approached — a   vast   actual   ring   of   stars 
138 


FreaHs  and  Oddities  139 

encircling  the  whole  of  the  visible  universe:  "a 
ground  plan  of  the  system."  To  the  casual 
observer  it  appears  only  as  a  hazy  band  of  light, 
brighter  in  some  places  than  in  others;  but  a 
little  closer  observation  will  show  that  the  ring 
is  irregular,  and  not  homogeneous.  It  is 
broad  and  diffuse  on  one  side,  and  narrow  and 
well  defined  on  the  other.  Also,  it  is  split  in 
two  by  a  great  rift  for  about  a  third  of  its  cir- 
cumference. More  careful  looking  will  show 
that  it  is  marvelously  complex  in  detail.  The 
telescope  only  can  show  its  true  beauty  and 
wonder.  Viewed  with  the  aid  of  even  a  small 
instrument,  the  Galaxy  is  seen  to  be  composed 
of  millions  upon  millions  of  stars  unevenly 
distributed  along  it  in  knots,  clusters,  streams, 
and  clouds,  and  in  some  places  with  the  effect  of 
almost  moving  swirls  of  light.  It  has  been 
estimated  that  the  Galaxy  contains  about 
thirty  stars  for  every  one  existing  outside  its 
borders. 

Many  of  the  first-magnitude  stars  seem  associ- 
ated with  this  great  "ground  plan  of  the  Uni- 
verse," and  some  of  the  bright  groups  of  stars, 
such  as  the  Pleiades  and  the  Hyades,  in  the 
constellation  Taurus,  appear  to  be  hung  from  it 


140    The  Essence  of  Astronomy 

like  pendants  from  a  necklace,  suspended  by 
faint  loops  from  the  main  ring. 

STAR  CLOUDS  are  found  almost  without 
exception  in  the  Milky  Way.  These  were  first 
photographed  by  Professor  Barnard,  and  not 
till  then  was  their  true  beauty  known.  The 
resemblance  to  the  clouds  of  our  atmosphere  is 
so  close  as  to  be  startling,  every  variety  of 
formation  being  clearly  portrayed.  It  must  not 
be  supposed,  however,  that,  despite  the  appear- 
ance, the  stars  of  these  clouds  are  literally 
crowded  together;  this  is  only  the  effect  of  the 
perspective  of  thousands  of  light-years.  It 
is  unquestioned,  nevertheless,  that  they  are 
much  nearer  to  each  other  than  the  Sun  and  its 
neighbors,  and  the  night  sky,  viewed  from  a 
planet  whose  sun  formed  a  member  of  such  a 
cloud,  would  be  of  a  glory  almost  inconceivable. 

There  are  two  great  exceptions  to  the  rule 
that  Star  Clouds  are  not  found  beyond  the 
limits  of  the  Galaxy.  These  are  the  "Magel- 
lanic  Clouds,"  two  "patches  of  hazy  light," 
looking  like  pieces  fallen  from  the  great  ring,  and 
of  essentially  the  same  structure  as  the  latter, 
so  much  so  that  it  is  only  their  separation  from 


FreaKs  and  Oddities  141 

it  which  makes  them  of  especial  interest.     They 
may   be   seen   only   from   southern   latitudes. 

STAR  SWARMS  are  also  characteristic  features 
of  the  Milky  Way.  They  are  merely  coarse 
Star  Clouds;  in  other  words,  their  component 
stars,  while  very  numerous,  retain  to  a  certain 
extent,  their  individuality,  and  do  not  merge 
together  in  a  misty  light  as  in  a  Star  Cloud. 
This  may  be  either  because  they  are  nearer  to 
us,  or  because  the  stars  are  larger  or  more  bril- 
liant than  those  composing  the  Star  Clouds,  or 
for  both  reasons. 

Some  of  these  can  be  seen  with  the  naked 
eye,  the  most  conspicuous  example  being  the 
"double-cluster,"  as  it  is  called,  in  constellation 
Perseus.  This  glows  only  as  a  tiny  patch  of 
light  to  the  eye  unaided,  but  viewed  through  a 
telescope  it  is  a  marvelous  sight.  The  illus- 
tration facing  page  144  is  from  an  excellent 
photograph.  The  reasons  for  these  groupings 
into  Star  Swarms  and  Star  Clouds  is  not  known, 
nor  has  any  tenable  theory  been  advanced. 

STAR  CLUSTERS  are  smaller  than  the  groupings 
just  mentioned,  and  the  term  is  properly  applied 


142    TKe  Essence  of  Astronomy 

only  to  those  which  show  a  globular  shape,  by  far 
the  most  characteristic  appearance.  They  are 
more  numerous  in  the  Milky  Way  than  outside 
its  boundaries,  but  many  are  found  with  no 
association  to  the  great  ring.  The  most  notable 
example  in  the  northern  hemisphere  is  the  great 
cluster  in  the  constellation  Hercules.  This  is 
visible  to  the  naked  eye  as  a  faint  glow  of  light. 
A  small  glass  will  show  its  character;  but  the 
astounding  live  beauty  of  it  is  apparent  only 
with  the  aid  of  a  large  telescope.  A  photograph 
cannot  show  the  wonders  of  a  cluster  for  the 
stars  at  the  center  are  lost  in  an  indistinguishable 
blur  (see  illustration  facing  page  160).  In 
fact,  this  effect  is  seen  to  a  lesser  extent  even 
when  viewing  the  group  through  a  telescope. 

Herschel  estimated  that  the  Hercules  cluster 
contains  about  fourteen  thousand  suns,  each, 
however,  probably  much  smaller  than  our  Sun; 
five  thousand  is  probably  more  nearly  correct. 
Marvelous  as  the  sight  is  to  us,  how  much 
more  beautiful  to  an  inhabitant  of  a  near  world ! 

Here,  as  with  the  Star  Clouds,  the  perspective 
deceives  us.  The  stars  are  not  packed  closely, 
but  are  probably  separated  by  millions  of  miles. 
However,  the  globular  shape  of  the  clusters,  and 


FreaKs  and  Oddities  143 

the  arrangement  in  more  or  less  curving  and 
radiating  lines  of  the  dispersed  stars  surrounding 
the  cluster  proper,  have  led  to  the  suggestion  that 
the  whole  is  the  result  of  an  explosion  of  a  vast 
sun.  The  well  supported  observation  by  Her- 
schel,  that  space  near  these  clusters  is  unusually 
vacant,  as  in  the  region  of  the  nebulae,  seems  to 
point  to  a  less  spectacular  origin,  namely  a 
gradual  drawing  together  of  the  neighboring 
stars  towards  the  central  condensation  of  a 
nebula. 

There  is  a  still  greater  globular  cluster  in  the 
southern  hemisphere,  in  the  Centaur,  which 
constellation  also  contains  the  Sun's  nearest 
neighbor.  This  is  a  replica  of  the  Hercules 
cluster  in  all  respects  except  size. 

STAR  STREAMS  are  simply  "drawn  out"  star 
swarms  or  clusters.  The  Galaxy  is  replete  with 
them,  but  no  true  examples  are  found  outside  its 
limits. 

The  "Coal  Sacks"  is  the  inadequate  name 
given  to  the  inexplicable  dark,  starless  gaps 
that  appear  here  and  there  in  the  radiance  of  the 
Milky  Way.  The  first  noticed,  and  best  known, 


144    XHe  Essence  of  Astronomy 

is  close  to  the  "Southern  Cross."  For  years, 
and  in  fact  at  the  present  date,  this  is  regarded 
with  a  superstitious  awe  by  many,  particularly 
the  sailors,  those  accustomed  watchers  of  the 
skies. 

Up  to  the  very  edge  of  this  dark  hole  the 
Milky  Way  is  extremely  bright,  and  then  is 
extinguished  abruptly.  One  faint  star  shines 
within  the  opening,  making  the  surrounding 
blackness  the  more  profound  by  contrast.  The 
shape  of  the  "Sack"  is  that  of  a  pear,  and  the 
dimensions  about  eight  degrees  by  five,  giving  an 
area  of  over  125  times  that  of  the  full  Moon. 
Photography  has  shown  that  the  southern  part 
of  this  is  not  as  vacant  as  it  seems,  but  the 
northern  part  is,  apparently,  bottomless  beyond 
imagination. 

There  is  another  well-known  "Coal  Sack"  in 
the  northern  hemisphere,  in  the  constellation 
Cygnus,  and,  curiously  enough,  adjacent  to  the 
part  of  that  constellation  called  the  "Northern 
Cross."  This  is  neither  so  large  nor  so  well 
marked  as  the  other,  but  has  rather  the  appear- 
ance of  a  veil  drawn  over  the  stars,  lending 
support  to  the  theory  that  in  this  direction  we 
are  looking  through  a  dark  nebula. 


THE    DOUBLE    STAR    SWARM    IN    PERSEUS 

This  is  visible  to  the  naked  eye,  and  evident  in  a  small  glass,  but  a  good  telescope  is  needed  to 

bring  out  its  great  beauty 
From  a  photograph  taken  at  the  Yerkes  Observatory 


FreaKs  and  Oddities  145 

In  the  constellations  Scorpio  and  Sagittarius 
are  a  number  of  small  "Coal  Sacks, "  none,  how- 
ever, comparable  in  size  with  the  two  large  ones 
mentioned. 

Again,  many  theories  have  been  advanced  to 
explain  these  strange  "windows  of  absolute 
night,"  as  Serviss  calls  them,  but  it  seems 
certain  that,  though  some  of  them  may  be  the 
result  of  the  interposition  of  dark  nebulae,  the 
majority  are  actually  holes  in  the  boundaries  of 
our  universe  through  which  we  gaze  beyond  into, 
as  far  as  man  can  know,  infinite  space. 

ABNORMAL   STARS 

There  are  in  the  skies  many  stars  apparently 
quite  different  from  the  normal  "e very-day" 
variety  as  seen  by  the  naked  eye. 

These  may  be  divided  into  the  following 
classes:  Double  and  Multiple  Stars,  Binary 
Stars,  Variable  Stars,  and  New  or  Temporary 
Stars. 

DOUBLE  STARS.  There  are  many  stars  seem- 
ingly single  points  of  light  to  the  naked  eye 
which,  with  the  aid  of  the  telescope,  may  be 
resolved  into  two  or  more  stars.  In  some  cases 
this  is  merely  the  result  of  perspective.  Two 


146    TKe  Essence  of  Astronomy 

stars  nearly  in  a  line  when  viewed  from  the 
Earth,  will,  although  billions  of  miles  apart,  one 
farther  away  from  us  than  the  other,  appear  side 
by  side  on  the  apparent  celestial  sphere.  In 
many  cases,  however,  the  stars  are  actually  close 
together. 

Many  of  these  double  stars  show  beautiful 
contrasts  in  color.  The  star  Beta  of  Cygnus, 
for  example,  can  be  separated  into  two  by  even 
a  small  glass,  the  primary  a  brilliant  topaz, 
and  the  smaller  star  a  wonderful  peacock  blue, 
while  Gamma  of  Andromeda  separates  into  an 
orange-colored  primary  and  a  smaller  star  that 
shines  like  an  emerald.  This  color  contrast  in 
double  stars  is  the  rule  rather  than  the  exception, 
but  some  of  the  famous  doubles  are  of  the  same 
shade.  Castor  of  the  Twins  divides  into  two 
blue  white  diamonds  of  almost  equal  brilliancy, 
and  Mizar,  the  star  at  the  bend  of  the  handle  of 
the  "Big  Dipper,"  and  little  61  of  Cygnus,  the 
star  whose  distance  was  first  measured,  both 
separate  into  pairs  of  nearly  the  same  color. 

MULTIPLE  STARS  are  not  nearly  so  numerous 
as  simple  doubles.  There  are  many  known, 
however.  The  sextuple  star  in  the  heart  of  the 


FreaKs  and  Oddities  147 

Orion  nebula  is  the  best  example.  A  small 
telescope  shows  this  as  four  stars  only,  placed  in 
the  form  of  a  trapezium,  but  a  large  instrument 
adds  two  more  to  the  group. 

The  small  naked-eye  star  Epsilon  of  Lyra  is 
just  separable  by  a  good  eye.  A  telescope  again 
divides  each  component,  showing  a  system  of 
four  stars.  The  observation  of  Double  Stars 
is  one  of  the  pleasures  within  the  grasp  of  the 
owner  of  even  a  small  instrument. 

BINARY  STARS  are  merely  double  stars  whose 
orbital  revolution  about  each  other  has  been 
proven.  They  are,  as  a  rule,  much  closer 
together  than  the  average  "double."  Castor, 
mentioned  above,  is  a  fine  example  of  a  binary 
system. 

Many  stars  that  defy  separation  with  a 
telescope  yield  their  secret  to  the  spectroscope 
and  are  proven  close  doubles.  These,  in  con- 
sequence, are  called  spectroscopic  binaries. 
Each  component  of  the  telescopic  double 
Mizar  of  the  Great  Bear,  also  mentioned 
above,  has  been  proved  to  be  a  double,  though 
no  telescope  will  ever  be  able  to  show^  the 
four  stars. 


148    THe  Essence  of  Astronomy 

A  VARIABLE  STAR,  or  merely  a  variable,  as 
it  is  more  commonly  known,  is  one  whose 
magnitude — that  is  its  brilliancy  as  viewed 
from  the  Earth — changes  from  time  to  time, 
these  changes  being  in  some  cases  slight  and 
in  others  very  great,  and  occurring  usually  in 
a  comparatively  regular  cycle. 

The  variables  are  generally  classified  as 
"short  period,"  those  whose  cycle  is  completed 
in  a  few  days  or  less,  and  "long-period,"  those 
which  require  more  time,  in  some  cases  even 
years. 

The  SHORT-PERIOD  VARIABLES  may  be  sub- 
divided into  those  whose  variation  is  caused 
by  a  partial  eclipse  of  the  brilliant  star  by  a 
companion  star,  a  close  binary,  the  two  re- 
volving about  each  other,  and  those  where  the 
variation  is  partially  due  to  some  real  change 
in  the  star  itself  which  alternately  blazes  up 
and  smolders. 

The  best  known  example  of  the  former  class  is 
the  star  called  "Algol,"  which  shines  in  the 
constellation  Perseus.  With  great  regularity, 
every  two  days  twenty  hours  and  forty-eight 
minutes,  the  light  of  Algol  undergoes  a  rapid 


Freaks  and  Oddities  149 

change,  first  decreasing  over  a  magnitude,  and 
then  regaining  its  full  brilliancy,  the  duration 
of  the  whole  phase  being  nine  hours  and  twenty 
minutes.  This  means  that  at  maximum  the 
star  gives  over  twice  as  much  light  as  it  does  at 
minimum.  This  peculiar  behavior  of  Algol  has 
been  long  known,  and  its  name,  signifying  "The 
Demon,"  shows  the  riddle  it  propounded.  By 
the  aid  of  the  spectroscope  this  riddle  was  solved, 
and  it  is  now  known  definitely  that  Algol  is  a 
system  of  two  stars,  one  bright,  the  other  com- 
paratively dark,  and  invisible  even  with  the  aid 
of  the  great  telescopes.  These  revolve  about 
each  other — that  is,  around  their  common  center 
of  gravity,  as  do  the  Earth  and  Moon — in  a  period 
exactly  equaling  the  cycle  of  Algol's  variation  in 
magnitude.  It  is  the  passage  of  the  dark  star 
between  us  and  Algol  which,  by  partially  eclips- 
ing the  latter,  cuts  off  from  us  much  of  its  light. 
From  spectroscopic  observations  of  the  move- 
ments of  the  bright  star  in  its  orbit,  it  is  possible 
to  estimate,  with  a  fair  approach  to  certainty  and 
accuracy,  that  Algol  is  about  1,000,000  miles 
in  diameter,  its  dark  companion  about  765,000 
miles  in  diameter,  or  slightly  larger  and  slightly 
smaller,  respectively,  than  our  own  Sun;  and 


150    THe  Essence  of  Astronomy 

that  the  bodies  are  about  2,250,000  miles  apart 
from  surface  to  surface. 

Constant  observation  has,  in  the  past  few 
years,  added  many  variables  to  this  "Algol 
type, "  some  of  which  are  almost  faultless  time- 
keepers. 

Another  well-known  variable  of  short  period 
is  the  star  Beta  of  the  constellation  Lyra. 
Unlike  that  of  Algol,  the  light  change  in  this  case 
is  not  a  simple  rise  and  decline.  The  brilliancy 
rises  to  a  maximum,  declines  about  halfway  to 
minimum,  rises  again  to  maximum,  and  then  sinks 
back  to  full  minimum,  the  total  cycle  requiring 
about  thirteen  days.  The  variability  of  this 
star  is  supposed  to  be  caused  almost  entirely  by 
the  mutual  eclipsing  of  each  other  by  two 
bright  stars  of  somewhat  different  size  revolving 
close  together.  This,  however,  does  not  explain 
all  of  the  details  of  the  variations,  and  stars  of 
the  Beta  Lyrae  type  are  still  among  the  unsolved 
problems  of  the  skies. 

The  other  types  of  short-period  variables  are 
severally  classified,  but  these  subdivisions  need 
not  be  discussed  here.  It  is  probable  that  the 
changes  of  all  are  due  to  a  great  extent  to  the 
close  revolution  and  eclipses  of  two  or  more 


TreaKs  and  Oddities  151 

stars,  in  some  cases  whirling  in  actual  contact 
with  each  other,  and  also  to  a  real  flaming  out  of 
the  stars  themselves. 

Our  Sun  has  recently  been  proved  to  be  a 
variable  of  short  period,  but  the  variation  is  so 
extremely  slight  as  to  be  measurable  by  only 
the  most  delicate  instruments.  This,  and  the 
fact  that  almost  all  variables  (where  the  con- 
stitution of  the  star  itself  causes  the  change)  are 
of  a  reddish  tinge,  some  long-period  variables, 
indeed,  being  veritable  rubies  in  color,  lends 
plausibility  to  the  theory  that  variability  is  a 
sign  of  advancing  stellar  age,  and  that  it  is 
caused  in  many  cases,  by  extreme  conditions 
similar  to  those  producing  the  solar  spots  and 
prominences. 

Another  theory  attempting  to  explain  varia- 
bility in  stars  is  based  upon  a  partial  or  grazing 
collision  of  two  stars.  In  such  occurrences  it  is 
argued  that  a  great  rift  or  valley  would  be  gouged 
out  of  each  star,  leaving  that  part  of  each  a  mass 
of  flaming  gases  raised  to  an  enormous  heat 
by  the  impact  and  friction  of  the  collision.  A 
secondary  effect  of  such  a  collision  would  be  the 
rotation  imparted  to  each  star  so  that  it  would 
sweep  through  the  skies  a  brilliant  beam  of 


152    THe  Essence  of  Astronomy- 
light  emanating  from  its  torn  side  as  does  a  light- 
house with  its  revolving  search-light. 

Of  LONG- PERIOD  VARIABLES  many  are  known. 
Some  of  these  pass  through  their  changes  with  a 
real  regularity,  while  others  are  so  erratic  that 
no  period  or  cycle  can  be  stated.  In  fact,  these 
irregular  variables  seem  to  scorn  any  degree  of 
constancy,  and  the  maximum  of  one  rise  may  fall 
far  short  of  previous  brilliancy.  The  extreme 
rapidity  with  which  some  of  these  freaks  "rise" 
is  astounding.  In  some  cases  only  three  or  four 
days  is  necessary  for  one  to  rise  to  a  maximum 
forty  times  more  bright  than  its  minimum. 

The  most  noted  long-period  variable  is  Mira 
"the  Wonderful"  of  the  constellation  Cetus. 
This  has  been  under  observation  for  over  three 
hundred  years,  and  is  the  first  of  these  stars  to  be 
recognized.  In  the  course  of  about  331  days 
Mira  changes  slowly  and  evenly  from  a  star  of 
over  second  to  one  of  less  than  ninth  magnitude, 
and  back  again;  so  that,  while  at  maximum  it  is 
one  of  the  most  conspicuous  stars,  it  is  for  over 
five  months  of  its  cycle  invisible  to  the  naked 
eye.  It  gives  at  maximum  about  J75  times 
as  much  light  as  at  minimum. 


FreaKs  and  Oddities  153 

By  careful  study  of  these  long-period  variables 
it  has  been  possible  to  classify  many  of  them 
according  to  their  "light-curves,"  and  con- 
tinued observation  and  spectroscopic  study  will 
without  doubt  discover  the  causes  for  all  the 
various  peculiarities  of  these  "freaks." 

NEW  STARS.  Occasionally  the  careful  ob- 
server of  the  skies  will  note  a  star  where  previ- 
ously none  was  to  be  seen,  and  which  is  not 
charted.  This  is  called  a  new  star,  or  techni- 
cally a  Nova. 

The  causes,  or  cause,  of  these  sudden  appear- 
ances, for  sudden  they  indeed  are,  is  not  defi- 
nitely known,  but  it  is  certain  that  they  are 
evidences  of  some  enormous  cataclysm.  It  is 
generally  accepted  that  a  Nova  is  the  result  of  a 
collision  of  some  sort  between  two  stars.  The 
main  result  of  such  a  collision,  whether  a  partial 
or  grazing  one,  or  a  head-on  meeting,  between 
any  bodies  traveling  at  the  speed  common  in 
the  universe,  would  be  the  transformation  into  an 
incandescent  mass  of  gas  of  all  the  directly 
colliding  matter,  which,  radiating  with  a  surpass- 
ing brilliance,  would  shine  to  us  as  a  star. 

Whether  these  collisions  took  place  between 


154    TKe  Essence  of  Astronomy- 
dark  stars  or  bright  stars,  or  a  bright  star  and 
a  dark  star,  the  result  would  be  the  same,  vary- 
ing only  in  intensity  according  to  the  size  and 
density  of  the  colliding  bodies. 

There  are  many  subdivisions  of  the  star- 
collision  theory,  and  certain  objections  to  it  are 
advanced,  all  of  which  are  of  too  technical  a 
nature  to  be  given  here.  The  dissenters  offer 
other  theories,  such  as  the  passage  through 
a  dark  nebula  by  a  star,  either  bright  or  dark, 
and  the  consequent  heat  and  incandescence 
from  friction;  the  collision  of  meteor  swarms; 
or  the  actual  explosion  of  a  star  by  atomic  dis- 
integration. This  last  suggestion,  which  science 
is  finding  not  so  improbable  as  it  sounds,  is  not 
the  most  pleasant  to  contemplate,  for  granting 
the  truth  of  it  admits  the  possibility  of  our  Sun 
not  denying  itself  as  long  as  we  Earth  dwellers 
should  prefer  an  explosive  debauch  of  this  kind, 
which  would  wreck  the  whole  Solar  System. 

New  stars  seem  to  pass  through  about  the  same 
general  changes.  They  flash  up  from  nothing 
to  widely  ranging  degrees  of  brilliancy,  some 
never  being  visible  to  the  naked  eye,  and  others 
being,  for  a  time,  the  most  conspicuous  of  the 
stellar  host.  Their  maximum  brightness  is  held 


FreaKs  and  Oddities  155 

for  a  comparatively  short  period,  and  they  then 
fade  slowly,  occasionally  blazing  up  briefly  as  if 
in  vain  attempts  to  retain  their  passing  glory. 
The  decrease,  however,  is  always  great,  and  a 
Nova,  as  a  rule,  either  entirely  disappears,  or 
remains  as  a  very  faint,  and  often  nebulous, 
star,  usually  a  variable. 

Novae  that  rise  to  great  magnitude  are  rare. 
This  is  unfortunate,  as  it  is  difficult  to  make 
proper  spectroscopic  study  of  faint  stars,  and  we 
must  look  to  the  spectroscope  to  obtain  the 
facts  required  to  explain  these  curiosities  of 
the  skies.  Theoretical  discussion  has,  as  we 
have  said,  produced  many  theories,  but  a  few 
more  opportunities  to  observe  brilliant  Novae 
will  very  possibly  remove  them  from  among  the 
unsolved  problems,  to  the  realm  of  fact. 

The  first  Nova  mentioned  was  that  observed 
by  Hipparchus  in  123  B.C. — and  its  sudden 
appearance  so  upset  his  notions  of  the  perma- 
nency of  the  stars  that  he  began  the  first  great 
star  catalog. 

The  most  famous  and  the  most  brilliant  on 
record  appeared  in  1572,  in  the  constellation 
Cassiopeia.  This  was  studied  by  Tycho  Brahe 
and  is  often  referred  to  as  Tycho's  star.  It  was 


156    THe  Essence  of  Astronomy 

at  one  time  so  bright  as  to  be  visible  in  daylight. 
There  is  now  absolutely  no  trace  of  it. 

Of  recent  years,  the  brilliant  Novae  have  been 
Nova  Auriga  and  Nova  Persei.  Both  of  these 
were  discovered  by  a  Dr.  Anderson  of  Edin- 
burgh, a  naked-eye  observer,  in  1892  and  1901 
respectively.  The  latter  was  extremely  bril- 
liant, and  rose  in  twenty-four  hours  from  total 
invisibility  (the  region  was  photographed  the 
day  before  it  was  seen  and  showed  no  trace  of 
such  a  star)  to  third  magnitude,  and  speedily 
attained  first  magnitude. 

It  was  from  this  star  that  a  nebula  seemed  to 
spread  with  amazing  velocity.  It  is  yet  un- 
known whether  this  nebula  was  actually  formed 
and  radiated  as  it  appeared,  or  whether  it  was 
an  existing  dark  nebula  into  which  the  star 
rushed,  thus  becoming  brilliant,  the  nebula  itself 
growing  visible  only  as  the  light  from  the  flaming 
star  rushed  outward.  The  next  brilliant  Nova 
may  throw  our  way  light  enough  to  reveal  the 
answer. 

There  have  been  many  other  new  stars  ob- 
served but  only  abou.t  nineteen  of  them  since 
the  time  of  Hipparchus  have  been  of  noticeable 
brilliancy.  Nowadays,  they  are  usually  found 


FreaKs  and  Oddities  157 

by  photography,  a  comparison  of  two  plates 
of  the  same  region  showing  immediately  any 
discrepancy  between  them,  and  such  discrep- 
ancies being  immediately  investigated.  A  gen- 
eral spectrographic  photograph  of  a  region  of 
the  skies  will  often  show  Novae,  as  they  have 
peculiar  spectra  easily  recognized  by  experts. 
There  is  nothing  which  so  stirs  the  astronomical 
world  as  the  discovery  of  a  brilliant  Nova. 


CHAPTER  XIX 

ASTRONOMICAL   INSTRUMENTS 

THE  more  important  instruments  used  in 
the  modern  observatories  are :  the  Telescope ;  the 
Spectroscope;  the  Photographic  Telescope;  the 
Meridian  Circle;  and  the  Astronomical  Clock. 
There  are  many  other  special  instruments  as, 
for  instance,  one  which  shows  the  amount  of 
light  reaching  us  from  an  object;  another  which 
estimates  the  amount  of  heat  received;  and  a 
number  of  attachments  for  the  telescope  which 
assist  in  making  delicate  measurements  of  the 
size  and  distance  apart  of  objects  viewed.  The 
description  of  these  special  instruments  would 
be  out  of  place  here. 

THE  TELESCOPE 

In  complete  form,  as  used  in  observatories, 
the  telescope  and  its  mounting  is  a  complicated 
structure.     The   principles   of   the   instrument 
can,  however,  be  readily  understood. 
158 


Astronomical  Instruments      159 

The  main  function  of  a  telescope  is  to  make 
distant  objects  look  near.  The  optical  appli- 
ances by  which  this  is  accomplished  are  very 
simple.  They  are  merely  lenses  of  glass,  ground 
with  great  perfection.  The  most  necessary  part 
of  the  instrument  is  a  means  of  collecting  light 
coming  from  an  object,  so  that  an  image  of  the 
latter  may  be  formed.  This  light-collecting  is 
done  either  by  passing  the  light  through  a  lens, 
or  by  reflecting  it  from  a  concave  mirror  of 
glass  or  metal.  It  may  be  seen,  therefore,  that 
there  are  two  main  classes  of  telescope:  the 
first  being  called  the  Refracting  Telescope,  or 
Refractor;  the  second,  the  Reflecting  Telescope, 
or  Reflector. 

The  REFRACTING  TELESCOPE.  The  lenses  of 
a  refractor  are  the  object-glass,  or  objective,  and 
the  eyepiece.  The  former  collects  the  light 
and  forms  an  image  of  the  object  at  its  focus, 
at  the  other  end  of  the  telescope  tube,  and  the 
latter  magnifies  this  image  as  desired.  Tele- 
scopes have  but  one  objective,  but,  as  a  rule, 
are  fitted  with  eyepieces  of  widely  ranging  mag- 
nifying power,  for  different  uses. 

The  objective  is  nearly  always  of  two  pieces 


160    XHe  Essence  of  Astronomy 

so  closely  put  together  as  to  seem  one.  This  is 
to  avoid  what  is  known  as  the  dispersion  of  a  lens. 
The  reader  probably  is  aware  that  ordinary  light 
is  composed  of  a  great  number  of  colors,  which 
can  be  separated  by  passing  the  light  through 
a  prism,  this  resulting  in  the  "rainbow"  effect 
known  as  a  spectrum.  In  fact  a  rainbow  is  simply 
the  Sun's  light  dispersed  by  the  countless  rain- 
drops acting  as  lenses.  It  is  most  essential  that 
an  object-glass  bring  to  a  clear  focus  all  the 
rays  coming  from  one  point  of  the  object  being 
viewed.  A  single  lens  will  not  do  this  since  it 
invariably  has  dispersive  power  and,  in  conse- 
quence, the  red  rays,  at  the  lower  end  of  the 
spectrum,  are  brought  to  one  focus,  while  the 
violet,  at  the  high  end,  are  concentrated  at 
another,  the  in-between  colors  each  having  a 
focus  of  its  own.  As  a  result,  the  image  is  not 
only  blurred  and  indistinct,  but  shows  an  un- 
natural colored  appearance.  Troublesome  as 
this  is  to  astronomers  in  connection  with  their 
telescopes,  it  is  nevertheless  a  great  blessing, 
for  it  is  upon  the  dispersive  power  of  glass  that 
the  wonderful  spectroscope  depends. 

In  1750,  Dollond  of  London  found  that,  by 
using  two  kinds   of   glass  of  widely   different 


THE    GREAT    SOUTHERN    STAR-CLUSTER    w    CENTAURI 

One  of  the  best  two  examples  of  a  true  star-cluster 

Note  the  streaming  of  small  stars  around  the  duster.      The  stars  of  the  cluster 
itself  are  too  numerous  to  be  counted,  or  even  to  be  separately 

distinguished  in  the  central  part 

Photographed  by  S.  I.  Bailey  at  the  South  American  Station  of 
Harvard  Observatory 


Astronomical  Instruments      161 

dispersive  powers,  it  was  possible  nearly  to 
correct  this  fault.  However,  even  in  the  best 
lenses  it  is  present  to  a  small  degree,  and  a  bright 
image  always  appears  surrounded  by  a  bluish 
radiance,  arising  from  those  violet  rays  not 
properly  brought  to  a  focus. 

The  objective  is  by  far  the  most  important 
part  of  the  telescope,  and  the  power  of  the 
instrument  is  limited  by  the  diameter  of  this 
lens,  the  aperture,  as  it  is  called.  In  order  to 
see  an  object  one  must  have  a  certain  amount 
of  light,  and  to  magnify  an  object  and  still  see  it 
at  its  original  brightness  requires  more  light. 
If  we  magnify  one  hundred  times,  we  need  ten 
thousand  times  the  light  to  keep  the  object  at  its 
natural  illumination.  It  is  possible  to  see  and 
study  most  celestial  objects  when  seen  far  below 
their  normal  illumination.  A  power  of  200 
can  be  used  upon  only  a  3-inch  telescope  with 
comfort  and  advantage  when  studying  the 
Moon,  for  instance,  although  a  3-inch  glass 
collects  only  225  times  more  light  than  the 
eye.  It  does  not  follow,  however,  that  be- 
cause a  lens  twenty  inches'  in  diameter  collects 
about  10,000  times  as  much  light  as  does  the 
naked  eye,  it  will  be  possible  to  use  a  mag- 


1 62    THe  Essence  of  Astronomy 

nification  of  10,000.      The  reason  for  this  is 
explained  later. 

An  objective  may  be  of  tremendous  light- 
gathering  power  and  yet  be  worthless,  for,  unless 
it  bring  properly  to  a  focus  the  rays  striking  it, 
the  image  formed  is  too  blurred  to  be  seen  with 
any  advantage.  The  focusing  quality  of  a 
lens  is  called  its  definition.  The  slightest  devi- 
ation from  the  proper  shape  of  a  lens  affects  its 
definition.  The  larger  the  objective  the  more 
serious  any  such  errors  become,  as  more  light  is 
incorrectly  focused. 

The  making  of  a  good  objective,  therefore,  is  a 
most  difficult  task.  The  manufacture  of  the 
glass  disks  from  which  it  is  to  be  ground  requires 
great  skill  and  patience,  since  the  slightest  flaw 
in  the  disk  or  the  smallest  inequality  of  density 
in  the  glass  renders  it  useless  for  the  purpose. 
Moreover,  the  grinding  of  a  lens  demands  al- 
most genius.  "A  generation  ago  there  was  but 
one  man  in  the  world  in  whose  ability  to  make 
a  perfect  object-glass  of  the  largest  size  astrono- 
mers everywhere  would  have  felt  confidence." 

The  image  formed  by  the  objective  is  always 
inverted.  This  makes  no  difference  from  an 
astronomical  viewpoint,  and  the  astronomical 


-Astronomical  Instruments      163 

eyepiece  makes  no  change  in  the  position  of  the 
image.  A  terrestrial  eyepiece,  however,  such  as 
is  used  in  spy-glasses,  has  two  additional  lenses 
which  re-erect  the  image,  and  it  is  seen  right  side 
up. 

The  eyepieces  of  a  telescope  are  nothing  more 
than  small  magnifying  glasses  somewhat  like 
those  used  by  a  watchmaker.  The  shorter  the 
eyepiece,  or  its  focal  length,  the  greater  the 
power.  They  are  usually  made  of  two  or  more 
lenses  arranged  in  various  combinations. 

The  REFLECTING  TELESCOPE  was  invented 
by  Newton  about  1670  in  consequence  of  his 
discovery  of  the  "decomposition  of  light,"  or 
dispersion,  which  caused  such  trouble  with 
refractors.  It  differs  from  the  refractor  mainly 
in  that  its  light-collecting  part  is  a  curved  mirror 
of  glass  or  polished  metal.  This  form  of  instru- 
ment has  many  advantages  over  the  refractor. 
It  is  free  from  the  trouble  caused  in  the  latter  by 
the  dispersion,  for  all  light  is  equally  reflected. 
It  can  also  be  made  larger  than  the  other,  as  it 
does  not  require  great  disks  of  glass  perfect 
throughout ,  and  does  not  cost  as  much  to  make. 
On  the  other  hand,  there  are  several  disadvan- 


1 64    THe  Essence  of  Astronomy 

tages  to  the  reflector  type.  It  is  impossible  to 
get  as  good  definition  as  with  a  refractor,  since 
an  error  in  a  lens  affects  the  image  only  one 
third  as  much  as  a  similar  error  in  a  mirror. 
There  is  also  constant  trouble  in  keeping  the 
polish  on  the  mirror  surface,  a  lack  of  polish 
meaning,  of  course,  a  corresponding  loss  of 
light.  There  is  also  more  difficulty  in  handling 
this  instrument;  and  finally,  the  reflector  gives 
a  fainter  image,  as  light  is  lost  by  the  double 
reflection  described  below. 

The  large  mirror  of  a  reflector  is  called  tech- 
nically the  speculum.  For  many  years  this  was 
made  of  an  alloy  called  speculum  metal.  Now, 
however,  glass  is  used,  and  the  reflecting  surface 
covered  with  a  thin  coating  of  polished  silver. 

It  will  be  readily  seen  that  there  are  practi- 
cal difficulties  in  using  a  reflector.  The  most 
obvious  one  is  that  the  rays  are  reflected  back 
in  the  direction  from  which  they  came,  and  to  see 
the  image  the  observer  must  look  into  the  mirror, 
so  to  speak.  If  he  does  this  directly,  his  head 
will  cut  off  from  the  mirror  a  large  portion  of  the 
light.  Some  means  of  obviating  this  difficulty, 
therefore,  is  necessary,  and  from  this  necessity 
have  come  the  four  forms  of  reflectors. 


Astronomical  Instruments      165 

The  Herschelian  form  is  practical  only  with 
large  instruments,  since  the  head  of  the  observer 
is  still  somewhat  in  the  way.  The  speculum  in 
this  form  is  not  set  squarely,  but  is  tilted  so  that 
the  image  is  formed  close  to  one  side  of  the  open 
end  of  the  tube.  This  method  is  no  longer  used. 

The  Cassegrainian  has  a  small  mirror  near 
the  focus  of  the  speculum  which  reflects  the  ray 
back  through  a  hole  in  the  speculum,  at  which 
hole  the  eyepiece  is  mounted.  There  are  but 
one  or  two  of  this  kind  in  use. 

The  Gregorian  is  like  the  above  save  for  a 
difference  in  the  small  mirror. 

The  Newtonian  form  is  by  far  the  most 
common,  and  is  the  most  convenient  to  use. 
Here  the  rays  are  reflected,  usually  by  a  prism, 
through  a  hole  in  the  side  of  the  tube. 

The  POWER  of  a  telescope  depends  upon  the 
combination  of  objective  and  eyepiece. 

Theoretically  by  using  an  eyepiece  of  suf- 
ficient power  we  can  get  any  magnification  we 
please,  but  there  are  many  difficulties  in  carry- 
ing the  magnification  of  an  instrument  beyond 
certain  limits.  First,  as  was  said,  there  is 
the  amount  of  light  necessary  to  see  an  object 


1 66    TKe  Essence  of  Astronomy 

under  great  magnification.  Second,  the  bad 
definition,  due  to  the  impossibility  of  bringing 
to  a  focus  all  the  light  rays,  grows  more  evident 
as  higher  powers  are  used.  Third,  and  most 
annoying  of  the  three,  is  atmospheric  dis- 
turbance. 

We  see  a  celestial  body  through  as  much 
atmosphere  as  we  do  an  object  on  the  Earth 
about  six  miles  away.  In  looking  at  such  an 
object  we  see  its  outline  blurred  and  softened. 
This  is  mainly  because  the  atmosphere  through 
which  the  light  rays  come  is  constantly  in 
motion,  thus  causing  an  irregular  refraction. 
Any  one  who  has  looked  across  a  field  on  a  hot 
day  must  have  noticed  the  tremulous  effect  of 
distant  objects  seen  through  the  rising  heated  air. 
This  is  simply  an  extreme  condition  of  the  always 
present  atmospheric  disturbances.  The  softened 
and  blurred  effect  of  an  object  thus  viewed  is  mag- 
nified in  a  telescope  as  many  times  as  the  object 
itself,  so  that  each  increase  of  magnifying  power 
increases  the  lack  of  definition.  It  is  to  avoid  as 
much  of  this  as  possible,  by  getting  above  the 
dense  sea-level  air,  that  observatories  are  placed 
at  high  elevations. 

Small    telescopes    are    limited,    in    the    high 


Astronomical  Instruments      167 

powers  they  may  use  effectively,  to  about  fifty 
diameters  to  each  inch  of  aperture.  That  is,  a 
telescope  with  an  objective  three  inches  in 
diameter  would  find  little  advantage  in  using 
an  eyepiece  magnifying  much  over  one  hundred 
and  fifty  times.  Very  good  object-glasses  will 
stand,  under  good  conditions,  powers  of  up  to  one 
hundred  diameters  per  inch  of  aperture — that 
is,  up  to  certain  limits.  The  great  Yerkes 
refractor,  for  instance,  with  an  aperture  of  forty 
inches,  should,  under  this  figuring,  be  able  to  use 
a  power  of  4000.  A  well-known  astronomer 
says,  however:  "I  doubt  whether  any  astronomer, 
with  any  telescope  now  in  existence,  could  gain  a 
great  advantage,  in  the  study  of  such  an  object 
as  the  Moon  or  a  planet,  by  carrying  his  magni- 
fication above  a  thousand,  unless  on  very  rare 
occasions  in  an  atmosphere  of  unusual  stillness." 
Some  of  the  telescopes  of  great  size  are: 

REFRACTORS  REFRACTORS 

Yerkes  40  inches          Lord  Rosse's  Diam.  6  feet 

Lick  36  Mt.  Wilson  Obs.  5     4 

Potsdam  31.5  Melbourne  4     ' 

Pulkowa    ]  Crossley  Reflector     "      3     " 

Nice  V  29.5-30     "  (Lick  Obs.) 

Paris          J 

Greenwich  28 

Vienna  27 

Washington  26.25 

Univ.  of  Virginia     26.25 

Lowell  Obs.  24 


1 68    TKe  Essence  of  Astronomy 

THE  MOUNTING  OF  A  TELESCOPE 

There  is  more  to  using  a  telescope  than  just 
pointing  the  instrument  at  the  object,  and  then 
observing  the  latter.  In  the  first  place,  suppose 
a  star  to  be  located  and  found  in  the  field,  as  is 
termed  the  small  patch  of  sky  seen  through  the 
telescope.  Before  a  proper  observation  can  be 
made,  the  star  has  apparently  moved  out  of  the 
field,  and  must  be  followed  by  turning  the  in- 
strument. This  apparent  motion  is  multiplied 
as  many  times  as  the  instrument  magnifies, 
and  is  in  consequence  quickly  noticeable. 

The  field  of  view  is  magnified  in  the  same  way 
so  that  it  covers  less  sky  area  as  the  power  is  in- 
creased. If  a  magnification  of  a  thousand  be 
used,  "it  would  be  as  if  we  were  looking  at  a  star 
through  a  hole  one  eighth  of  an  inch  in  diameter 
in  the  roof  of  a  house  eighteen  feet  high.  If  we 
imagine  ourselves  looking  through  such  a  hole 
and  trying  to  see  such  a  star,  we  shall  readily 
realize  how  difficult  will  be  the  problem  of  finding 
it  and  of  following  its  motion"  (Newcomb).  - 

It  is  to  overcome  these  difficulties  that  the 
mounting  of  a  telescope  is  devised;  by  "mount- 
ing" we  mean  the  whole  system  by  which  a 


Astronomical  Instruments      169 

telescope  is  pointed  at  a  star  and  made  to  follow 
it. 

In  principle  the  mounting  is  as  follows: 

There  are  two  axes  set  at  right  angles  to  each 
other.  The  main  axis,  the  polar  axis,  is  placed 
directly  parallel  to  the  axis  of  the  Earth,  and 
points  therefore  to  the  celestial  poles.  The 
other  axis,  the  declination  axis,  turns  in  a 
"sleeve,"  which  is  fastened  rigidly  at  right 
angles  to  the  polar  axis,  like  the  cross  of  a  T. 
On  this  second  axis  is  mounted  the  telescope. 
When  the  telescope  is  "set"  on  a  star,  the  polar 
axis  is  driven  by  clockwork,  so  that  it  re- 
volves in  an  opposite  direction  to  the  Earth's 
rotation  and  at  just  the  same  rate.  The  de- 
clination axis  and  the  telescope  are  turned  with 
it,  and  therefore  the  effect  of  the  Earth's  rota- 
tion is  neutralized  and  the  star  remains  in  the 
field. 

The  declination  axis  allows  the  telescope  to 
turn  upon  it  in  a  north  and  south  direction 
only,  but  as  the  polar  axis  revolves  in  an  east 
and  west  direction,  the  combination  enables  the 
observer  to  point  the  instrument  in  any  desired 
direction. 

This  form  of  mounting  is  called  the  equatorial. 


170    TKe  Essence  of  Astronomy 


The  accompanying  diagram  will  help  to  make 
it  clear. 

There  are  two  methods  by  which  stars  are 

"found,"  as  it  is 
called,  or  brought 
into  the  field.  Every 
telescope  has  what  is 
called  a  finder;  this 
is  a  smaller  telescope 
mounted  rigidly  on 
the  larger,  exactly 
parallel  to  it.  This 
finder  is  of  low  mag- 
nifying power  and 
has  in  consequence  a 
large  field.  By  sight- 
ing along  the  tele- 
scope it  can  be 
pointed  in  the  direc- 
tion desired,  so 
closely  that  the  ob- 
ject will  be  in  the  field  of.  the  finder.  The  tele- 
scope is  then  moved  until  the  object  is  in  the 
center  of  the  field  of  the  finder,  and  it  will  be 
then  in  the  field  of  the  main  telescope. 

However,  as  most  objects  an  observer  studies 


Pillar 

Polar  Axis 

Sleeve  of  Declination  Axis 

Declination  Circle 

Hour  Circle 

Counter-weight 

Telescope  Tube 


Astronomical  Instruments      17! 

are  too  faint  to  be  visible  with  the  naked  eye, 
and  even  with  the  finder,  other  assistance  is 
needed.  The  position  of  a  star  is  marked  on  the 
celestial  sphere  in  right  ascension,  and  declination 
corresponding  to  longitude  and  latitude  on  the 
Earth,  and  on  the  axes  of  the  telescope  are 
mounted  graduated  circles  (see  diagram  on 
page  170).  The  declination  circle  has  marked 
upon  it  degrees  and  fractions  of  a  degree,  so  as 
to  show  the  declination  of  the  spot  on  the  sky 
at  which  the  telescope  is  pointed.  The  hour- 
circle  is  divided  into  twenty-four  hours,  and 
each  of  these  again  into  sixty  minutes,  since 
right  ascension  is,  for  convenience,  measured 
in  time,  not  degrees.  To  find  a  star,  the  tele- 
scope is  set  at  the  proper  declination,  and 
after  consulting  the  sidereal,  or  ' 'star- time, " 
clock  a  simple  calculation  shows  the  proper 
"time"  on  the  hour-circle.  The  telescope  is 
set  accordingly,  the  clockwork  started,  and 
the  object  will  remain  in  the  field.  This 
may  seem  complicated,  but  it  is  a  very  simple 
process. 

The  circles  of  a  telescope  must,  of  course,  be 
very  accurately  set  and  marked.  The  best  ones 
are  of  silver,  and  the  markings  are  so  fine  that  a 


I72    TTHe  Essence  of  Astronomy 

magnifying  glass  is  necessary  to  read  the  lines 
and  figures  upon  them. 

The  diagram  shows  only  the  refractor  type; 
there  is  no  difference  in  the  principle  of  the 
equatorial  mounting  of  a  reflector. 

Needless  to  say,  no  equatorial  mounting  can 
be  quite  satisfactory  unless  permanently  set  on 
a  firm  pillar.  Small  portable  instruments  are 
sometimes  equatorially  mounted.  In  such  cases 
it  is,  of  course,  necessary  to  point  the  polar  axis 
as  carefully  as  possible  at  the  celestial  poles 
before  the  telescope  is  in  proper  position  to  use. 

The  large  telescopes  are  most  complex  struc- 
tures. As  a  rule,  they  have  more  than  one  finder, 
the  sizes  varying,  and  because  of  their  great 
weight  the  problem  of  handling  them  is  great. 
However,  this  problem  has  been  so  satisfactorily 
solved  that  the  observer  at  a  well-equipped 
telescope  " presses  the  button,"  and  the  rest  is 
done  for  him  by  motors  and  clockwork. 

THE   SPECTROSCOPE 

The  spectroscope  is  an  instrument  for  analyz- 
ing light.  This  sounds  more  as  if  it  belonged  in 
the  laboratory  of  a  physicist  than  in  the  obser- 


.Astronomical  Instruments      173 

vatory  of  an  astronomer.  It  is,  however,  by  its 
marvelous  aid  that  more  has  been  discovered 
about  the  universe  and  its  individual  parts  than 
it  seemed  possible  for  man  to  learn.  Without 
doubt,  its  wizard-like  reading  of  the  secrets  of 
the  cosmos  place  it  as  the  foremost  wonder  of 
the  world.  The  value  of  the  spectroscope  is 
just  barely  being  recognized.  Not  until  about 
1860  was  its  analytical  eye  turned  upon  the 
heavens  with  a  proper  understanding  of  the 
great  knowledge  which  it  could  reveal,  although 
the  first  principle  of  spectroscopic  analysis, 
"decomposition  of  light,"  was  discovered  by 
Newton. 

This  first  principle  is  commonly  known,  and  is, 
as  mentioned  in  the  paragraphs  upon  lenses,  that 
white  light  is  composite,  consisting  of  various 
colored  lights  which  are  spread  out,  or  dis- 
persed, when  passed  through  a  prism  of  glass  or 
some  refracting  material. 

The  sensation  of  light  is  produced  when  the 
retina  of  the  eye  is  struck  by  waves  or  vibrations 
in  the  ether,  which  pervades  all  matter,  provid- 
ing these  waves  move  with  the  proper  speeds. 
The  slow  light  waves  produce  the  sensation  of 
red  light,  the  most  rapid  that  of  violet,  and 


174    THe  Essence  of  Astronomy 

between  these  two  the  various  "rainbow" 
color  sensations  are  the  result  of  various  speeds  of 
waves.  White  light  is  composed  of  all  these 
various  wave  lengths  mixed  one  with  the  other, 
somewhat  similar  to  the  way  great  ocean  waves 
are,  on  a  windy  day,  covered  by  a  multitude  of 
smaller  or  shorter  ones. 

When  a  beam  of  light  is  passed  through  a 
prism,  it  is  separated  into  these  colors,  each  class 
of  wave  length  being  refracted,  or  bent  to  a 
different  degree,  but  all  of  each  class  invariably 
undergoing  the  same  amount  of  refraction,  and  a 
"rainbow"  effect  results,  with  the  red  at  one 
end  and  the  violet  at  the  other.  When  the 
original  light  is  passed  through  a  narrow  slit,  an 
even  band  of  colors  is  obtained;  this  is  called  a 
spectrum. 

These  colors  are  not  sharp  and  distinct,  but 
shade  by  perfect  gradation  each  into  the  next, 
so  that  it  is  barely  possible  to  say  where  one 
begins  and  the  other  ends. 

The  spectroscope  is  designed  to  allow  the 
light  to  pass  first  through  a  slit  and  then  through 
a  prism,  or  series  of  prisms  where  a  wider  dis- 
persion is  desired.  A  closely  ruled,  reflective 
glass  "grating,"  giving  the  same  effect,  is  now 


Astronomical  Instruments     175 

largely  used  instead  of  a  prism.  The  spectrum 
is  then  observed  through  a  small  "view- tele- 
scope," or  photographed. 

Briefly,  the  principles  of  spectroscopic  analysis 
are: 

Different  forms  of  matter  give  different 
spectra.  Every  spectrum  is  crossed  by  certain 
lines,  each  line,  or  series  of  lines,  being  produced 
by  one  "element"  and  by  no  other;  and  these 
lines  always  appear  in  the  same  relative  posi- 
tions. A  skilled  spectroscopist  is  able  to  say 
immediately  upon  seeing  a  clear  spectrum  that  it 
was  made  by  light  coming  from  specific  materi- 
als, for  a  knowledge  of  what  substances  pro- 
duce certain  lines  has  been  built  up  by  actual 
tests  in  the  laboratories.  In  connection  with 
this,  it  is  interesting  to  note  that  helium  was 
discovered  in  the  Sun  many  years  before  it 
was  found  on  the  Earth.  Its  lines  were  present 
in  the  solar  spectrum,  but  not  until  helium  had 
been  chemically  reduced  was  it  possible  to 
produce  artificially  a  spectrum  showing  those 
lines. 

Light,  as  has  been  shown,  laughs  at  distance, 
and  traverses  billions  of  miles  unchanged.  It 
makes  no  difference,  therefore,  whether  it  is  a 


176    THe  Essence  of  Astronomy 

star  many  light-years  away  or  a  lamp  a  few 
feet  off  which  is  examined  by  the  spectroscope. 
As  long  as  there  is  enough  light  to  give  a  spec- 
trum, analysis  is  equally  possible. 

The  spectroscope  is  also  used  to  measure  star 
velocities  in  our  line  of  sight — that  is,  coming 
toward  or  receding  from  us.  If  a  star  is  ap- 
proaching us,  the  spectroscope  "runs  into" 
more  than  a  normal  number  of  light  waves  per 
second,  and  all  the  spectrum  lines  of  that  star 
are  shifted  toward  the  violet  or  rapid  end  of  the 
spectrum;  and,  if  the  star  be  receding  from  us, 
the  shift  is  toward  the  red  or  slow  end,  since 
then  less  than  normal  the  number  of  waves  reach 
the  instrument.  By  this  shift,  the  speed  of  the 
star's  motion  can  be  closely  estimated. 

By  this  principle  are  observed  the  binary 
stars  which  are  too  close  together  to  be  separated 
visually.  If  two  stars  revolve  in  regular  orbits 
about  each  other,  each  will  first  approach  and 
then  recede  from  us,  in  the  period  of  one  revolu- 
tion, and  the  spectrum  will  show  a  double  system 
of  lines,  first  separating,  then  closing  together 
till  they  merge,  and  then  separating  again  in 
the  opposite  direction. 

Nowadays  the  spectroscope  is,  as  a  rule,  used 


Astronomical  Instruments      17? 

not  visually  but  photographically,  and  the 
spectra  are  studied  and  measured  at  leisure. 
As  in  other  branches  of  observational  astronomy, 
the  use  of  the  camera  has  been  of  inestimable 
value  in  spectroscopy. 

For  obtaining  spectra  of  stars,  nebulae,  and 
faint  objects  in  general,  the  spectroscope  is 
attached  to  the  eyepiece  end  of  a  telescope  that 
it  may  take  advantage  of  the  light-collecting 
power  of  the  large  instrument. 

The  writer  wishes  that  more  space  could  be 
devoted  here  to  this  marvelous  instrument  and 
its  use,  for  we  already  owe  to  it  much  of  our  most 
interesting  knowledge  of  astronomy,  and  it  is 
unquestionably  by  an  extension  of  spectro- 
scopic  work  that  the  answers  will  be  found  to 
many  as  yet  unsolved  problems. 

THE   PHOTOGRAPHIC  TELESCOPE 

As  has  been  stated  in  the  previous  pages, 
astronomers  are  turning  more  and  more  to  the 
photographic  plate  as  a  means  of  not  only 
recording  observations  for  further  study,  but 
also  of  making  observations  beyond  the  limits 
of  eyesight. 

12 


178    THe  Essence  of  Astronomy 

The  photographic  plate  is  peculiarly  sensitive 
to  rays  of  light  which  are  too  rapid  for  the  eye 
to  grasp,  and  so,  though  an  ordinary  telescope 
will  serve  the  purpose  fairly  well,  the  instrument 
for  photographic  use  is  given  an  objective  espe- 
cially designed  to  focus  these  rapid  rays.  As  an 
ordinary  camera,  mounted  equatorially  so  as  to 
follow  the  stars,  will,  with  a  few  minutes'  ex- 
posure, record  more  stars  than  can  be  seen  by 
the  naked  eye,  it  may  readily  be  imagined  what 
a  long  exposure  with  a  photographic  telescope 
will  do.  No  eye  can  see  the  wonderful  nebu- 
losity in  the  Pleiades,  as  shown  in  the  frontis- 
piece, nor  one  half  the  extent  of  the  great  Orion 
nebula;  and  while  a  star  of  about  the  zyth  mag- 
nitude is  at  the  visual  limit  of  our  telescopes,  the 
camera  plate  "  sees  "  down  to  the  2Oth  magnitude. 
Particularly  in  the  study  of  comets  and  nebulae, 
has  the  sensitive  plate  proved  of  very  great  and 
growing  value;  while  in  recording  solar  disturb- 
ances and  in  charting  the  stars  it  is  almost 
indispensable.  Only  in  planetary  work  does  it 
fail  to  equal  direct  vision,  and  here  the  fine  sur- 
face detail  desired  is  blurred  and  lost  because  of 
the  atmospheric  disturbances  during  the  ex- 
posure. 


Astronomical  Instruments      179 

There  are  moments  when  the  atmosphere  is 
almost  absolutely  steady  and  clear.  When  ob- 
serving visually,  such  a  moment  need  be  but  a 
fractional  part  of  a  second  for  the  eye  to  catch  a 
sufficient  glimpse  of  even  the  finest  detail.  The 
photographic  plate,  however,  needs  at  least  a 
couple  of  seconds  to  receive  and  record  an  im- 
pression of  faint  detail.  It  is  for  this  reason 
that  in  planetary  work  the  eye  is  still  superior 
to  photography.  Further  improvements  in  the 
sensitizing  of  plates  will  unquestionably  over- 
come the  present  difficulties. 

Except  for  the  specially  figured  objective,  and 
the  fact  that  it  is  of  shorter  focus  so  that  a 
greater  field  may  be  taken  in,  the  photographic 
telescope  differs  in  no  basic  way  from  the  or- 
dinary refractor.  A  refractor  is  equally  good  for 
photographic  and  visual  work,  as  all  rays  are 
equally  reflected,  though  unequally  refracted. 

THE  MERIDIAN  CIRCLE 

This  is  a  telescope  used  in  determining  the 
exact  position  of  the  celestial  bodies.  It  is 
especially  mounted  so  that  its  only  motion  can  be 
north  or  south,  along  the  meridian.  If  it  points 
exactly  south,  it  can  be  turned  on  its  axis  until 


i8o    THe  Essence  of  Astronomy 

the  line  of  sight  passes  through  the  zenith  and 
still  farther  through  the  pole  to  the  north 
horizon ;  but  it  cannot  be  turned  a  hair's  breadth 
east  or  west.  In  the  field  of  this  instrument 
is  a  fine  "spider  line, "  exactly  marking  the  me- 
ridian. There  is  also,  as  a  rule,  a  series  of  lines 
flanking  the  one  actually  on  the  meridian. 

The  right  ascension  of  a  star  is  the  same  as  its 
sidereal  time  of  crossing  the  meridian,  so  that 
the  problem  of  finding  the  former  is  theoreti- 
cally simple.  The  sidereal  clock  is  started,  set 
exactly  at  the  correct  sidereal  time,  the  tele- 
scope of  the  meridian  circle  pointed  at  where 
the  star  will  cross  its  field,  and  the  exact  moment 
when  the  star  passes  the  meridian  is  noted.  The 
time  by  the  sidereal  clock  then  shows  the  star's 
right  ascension.  This  indeed  should  be  simple, 
but  unfortunately  there  are  irregularities  to  be 
reckoned  upon.  It  is  impossible  to  make  a  clock 
of  the  exactness  required  by  an  astronomer,  and 
there  is  always  some  slight  error  in  the  placing 
of  the  meridian  instrument.  There  is  also  the 
error  of  personal  equation  to  be  considered,  no 
two  people  seeing  and  acting  in  just  the  same 
time,  so  that  where  one  observer  will  record 
a  star's  passage  as  too  early,  another  will  do  so 


Astronomical  Instruments      181 

too  late.  Allowances  must  be  made  for  all  these 
possible  errors,  and  therefore,  the  determination 
of  a  star's  right  ascension  is  really  a  difficult 
matter. 

The  decimation  of  a  star  is  read  by  micro- 
scopes from  very  carefully  ruled  circles  that  turn 
with  the  telescope.  There  is  also  the  possibility 
of  instrumental  error  here,  but  the  personal 
equation  is  eliminated,  since  the  reading  need  not 
be  made  at  a  precise  moment,  as  is  necessary  in 
noting  the  transit  of  a  star  across  the  meridian. 

In  speaking  of  errors  in  recording  the  time  of  a 
transit,  but  tiny  fractions  of  a  second  are  meant. 
Most  observatories  use  a  chronograph  to  record 
this  time.  The  chronograph  consists  of  a 
revolving  cylinder,  covered  with  paper  upon 
which  rests  a  pen  point,  so  that,  as  the  cylinder 
revolves,  the  pen  traces  a  line  upon  the  paper. 
The  paper  is  so  connected  by  an  electric  current 
passing  through  the  clock,  and  through  a  key 
held  by  the  observer,  that  every  beat  of  the 
clock  and  every  pressure  on  the  key  make  a 
notch  or  dot  in  the  pen  line.  When  the  observer 
sees  a  star  on  the  spider  line  in  his  instrument  he 
presses  the  key,  and  the  position  of  the  notch 
thus  made  in  the  pen  trace,  between  two  notches 


1 82    TKe  Essence  of  Astronomy- 
made  by  the  clock,  gives  the  exact  moment  at 
which  the  key  was  pressed. 

It  is  from  these  observations  that  the  astrono- 
mers figure  and  give  out  the  correct  standard 
time  by  which  the  clocks  and  watches  of  the 
whole  world  are  set. 

So  difficult  is  it  to  secure  and  obtain  the  neces- 
sary accuracy  of  all  parts  of  a  meridian  circle 
and  its  accessories,  that  the  "meridian  chamber" 
of  an  observatory  is  the  "holy  of  holies"  to  the 
staff,  and  only  the  most  favored  visitors  gain 
admittance  there. 

THE  ASTRONOMICAL   OR   SIDEREAL   CLOCK 

This  is  merely  a  most  accurate  timepiece,  of 
pendulum  action,  regulated  to  run  "star-time," 
the  day  of  which  is  about  three  minutes  and 
fifty-six  seconds  shorter  than  the  standard 
civil  day.  The  sidereal  day  is  the  exact  time 
the  Earth  requires  to  complete  one  full  revolu- 
tion ;  while  the  standard  day  is  the  average  time 
between  successive  solar  noons,  or  returns 
of  the  Sun  to  the  meridian ;  this,  because  of  the 
Earth's  revolution  about  the  Sun,  requiring 
somewhat  more  than  one  complete  rotation  of 
our  globe. 


Astronomical  Instruments      183 

A  sidereal  clock  gains  on  our  ordinary  clock 
about  two  hours  a  month,  or  twenty-four  hours 
in  a  year.  Therefore,  in  a  year  of  365  days,  the 
Earth  has  completed  366  rotations,  and,  in  a 
leap  year,  367. 


CHAPTER  XX 

CHRONOLOGY 

THE  science  of  Astronomy  is  without  doubt  the 
oldest  of  the  true  sciences.  Its  early  history, 
like  that  of  all  histories,  is  lost  in  the  depths  of 
time,  but  the  records  of  the  ancients  carry  it 
dimly  to  about  five  thousand  years  ago.  These 
records,  moreover,  are  such  that  they  demon- 
strate a  close  study  for  centuries  previous. 

Unquestionably  the  rising  and  setting  of  the 
Sun  and  the  Moon,  and  the  phases  of  the  latter, 
the  most  evident  of  celestial  phenomena,  must 
have  caused  a  wonderment  and  thought  at  the 
very  dawn  of  human  intelligence.  From  that 
first  wonderment  to  the  first  astronomical 
records  is  a  time  measured  only  by  conjecture. 

The  earliest  records  known  are  those  of  the 
Chinese.  Then  come  those  of  the  Babylonians 
and  the  Egyptians.  It  is  interesting  to  note 
that  from  some  of  the  Babylonial  observations, 
inscribed  upon  baked  bricks  lately  excavated,  it 
184 


Chronology  185 

has  been  possible  to  obtain  values  of  some  of  the 
Moon's  motions  accurate  enough  to  be  of  real 
assistance  in  a  recent  revision  of  certain  lunar 
tables. 

Not  till  the  time  of  the  later  Greek  philoso- 
phers, however,  do  we  get  connected  data.  It 
was  among  the  last  mentioned  that  real  obser- 
vational astronomy  began.  That  the  Greek 
nature  philosophers  devoted  much  thought  to 
the  structure  of  the  Universe  is  shown  by  the 
fact  that  we  can  pick  from  their  theories  enough 
of  what  we  now  know  to  be  true  to  give  a  fairly 
accurate  description  not  only  of  the  Solar  Sys- 
tem but  the  Universe  at  large.  Copernicus,  who 
hundreds  of  years  later  evolved  the  true  basic 
plan  of  the  Solar  System,  credits  not  himself,  as 
the  originator  of  the  theory,  but  Pythagoras 
(B.C.  569-470). 

Unfortunately  for  the  progress  of  the  science, 
these  early  philosophers  propounded,  with  their 
average  one  true  theory  apiece,  so  man}7  absurd 
conceptions  that  the  conservative  thinkers  chose 
to  remain  supporters  of  a  theory  confirmed,  or 
apparently  confirmed,  by  what  they  themselves 
saw.  It  is  for  this  reason  that,  though  great 
strides  were  made  in  observational  work,  a 


1 86    THe  Essence  of  Astronomy 

marvelously  ingenious,  complicated,  and  sadly 
false  theory  of  the  system  of  the  Universe  was 
accepted,  and  held  sway  for  centuries,  being 
superseded  only  by  the  present  so-called  Co- 
pernican  Theory  after  the  great  Kepler  had 
"smoothed  out  the  wrinkles"  in  this  latter. 

Through  the  Middle  Ages,  the  false  theory, 
evolved  by  Ptolemy,  was  believed  in  so  tena- 
ciously that  the  world  at  large  refused  to  accept 
the  true  facts  when  they  were  presented.  The 
removal  of  the  Earth  from  the  center  of  the 
Universe  to  the  insignificant  position  of  an 
attendant  upon  a  body  supposedly  created 
merely  to  give  light  and  heat  to  the  Earth  was 
too  much  for  humanity  to  admit.  Indeed,  much 
was  done  to  stamp  out  this  new  be-littling  doc- 
trine. History  sadly  points  to  Galileo  as  an  old 
man  denying,  under  threat  of  torture,  the  cosmic 
system  he  knew  to  be  true,  and  even  more 
sadly  to  Bruno,  previously  burned  at  the  stake 
for  refusing  to  abandon  such  heretical  beliefs. 

From  then  on,  however,  the  path  of  Astron- 
omy has  been  broad  and  clearly  lighted,  Kepler 
and  the  mighty  Newton  being  responsible,  so  to 
speak,  for  the  larger  part  of  the  illumination. 
It  is  upon  Kepler's  demonstration  of  the  true 


Chronology  187 

motions  of  the  planetary  bodies  (and  would  it 
were  possible  to  devote  many  pages  here  to  this 
greatest  of  all  astronomical  achievements)  and 
Newton's  magical  demonstration  of  the  universal 
rule  of  that  Gravitation,  known  since  early  times, 
that  modern  Astronomy  stands. 

Previous  to  the  time  of  Galileo,  astronomical 
instruments  were  of  the  most  primitive  and 
crude  construction,  and  were,  in  consequence,  of 
feeble  assistance.  After  the  invention  of  the 
telescope,  and  its  improvement  by  Galileo,  these 
instruments  have  evolved  with  marvelous  speed, 
until  now,  with  the  amazingly  accurate  measur- 
ing attachments  to  the  telescope;  that  greatest 
wonder  of  all  the  ages,  the  spectroscope,  the 
analyzer  of  light;  and  the  photographic  plate, 
that  magic  recorder  of  the  faintest  impression  of 
light,  it  seems  that  we  have  reached  the  limit  of 
the  science  of  optics.  However,  that  some  new 
and  still  more  marvelous  improvement  or  in- 
vention will  appear  seems,  from  analogy,  to  be 
almost  certain.  Many  times  before  has  Man 
been  at  the  apparent  limit  of  his  abilities,  only 
to  step  boldly  and  successfully  beyond. 


Directly  following  is  a  chronology  of  the  main 


1 88    THe  Essence  of  Astronomy 

events  of  Astronomy.  For  the  first  part  it  is 
necessarily  general  and  broad;  but  from  the 
time  of  Copernicus  to  the  present,  it  is  accurate 
according  to  the  accepted  historical  dates. 

CHRONOLOGICAL   TABLE 

Previous  to  A  few  ancient  records  have  come  down  to 
the  sth  USt  which  demonstrate  the  fact  that  even  in 
B  c  those  remote  years  astronomy  was  by  no 

means  a  new  science. 

There  are  reports  of  celestial  spheres  made 
in  China  in  the  2Qth  century  B.C.  and  another 
in  the  23d  century  B.C. 

It  is  said  that  in  2513  B.C.  the  Emperor 
Chueni  of  China  recorded  a  conjunction  of 
five  planets,  and  that  Yao  gave  instructions 
to  his  astronomers  to  determine  the  positions 
of  the  Equinoxes  and  the  Solstices;  also  that, 
at  this  time,  the  calendar  year  was  of  365 
days  with  an  added  day  every  four  years,  as 
in  our  present  calendar. 

In  2159  (?)  B.C.  two  Chinese  Royal  As- 
tronomers were  put  to  death  for  neglecting 
to  predict  an  eclipse. 

V<-^Tlecords  have  been  found  of  comets  and 
eclipses  running  back  to  1640  B.C. 

The  Saros  or  eclipse  cycle  used  by  the 
Egyptian  and  Chaldeans. 

Thales  (639-546  B.C.),  the  Greek  nature 
philosopher,  explains  the  phases  of  the  Moon, 
and  measures  the  angular  diameter  of  the 
Sun. 

Parmenides  shortly  afterward  declares  the 
Earth  to  be  a  sphere. 


Chronology  189. 

Pythagoras    (569-470    B.C.)    deduces    the  5th~ Century 
theory  of  planetary  motions,  which  Coperni-       B.C. 
cus  later  revised. 

Meton  (432  B.C.)  introduces  calendar 
reform.  The  Metonic  cycle  is  still  used  to 
determine  the  date  of  Easter. 

Democritus  (470-?  B.C.)  evolves  true 
conception  of  the  Universe. 

Heraclitus  supposes  Earth  to  rotate  upon 
axis. 

Callipus  (330  B.C.)  corrects  eclipse  cycle.      4th  Century 

Aristarchus      (320-250      B.C.)      measures         B>c* 
distance  of  the  Sun. 

Eratosthenes     (276-196     B.C.)     measures   2d  Century 
Earth    and  inclination   of   the  axis  to  the  B.C. 

ecliptic. 

Aristillus  and  Timocharis  fix  positions  of 
zodiacal  stars. 

Hipparchus  (190-120  B.C.)  founds  real 
.observational  astronomy;  again  measures 
inclination  of  axis;  remeasures  length  of  year; 
computes  lunar  tables;  observes  first  recorded 
"new  star";  makes  first  great  star  chart; 
discovers  precession  of  equinoxes;  invents 
trigonometry. 

Ptolemy  (139-161)  formulates  his  theory  of   2d  Century 
celestial  motions.  A-D- 

Alexandrian    library    burned    by    Caliph  7th  Century 
Omar. 

Moon's  variation  noted  by  Abul  Wefa.        loth  Century 

Ulugh-Begh  makes  first  star  catalogue  sinceisth  Century 
that  of  Hipparchus. 

Copernicus  born.  1473 

Astrolabe  invented  by  Behaim.  1480 

Copernicus    publishes    De    Revolutionibus  1543 

Orbium  Celestium,  his  great  work,  recalling 


190    THe  Essence  of  Astronomy 

as  a  basis  the  theory  of  Pythagoras,  thus 
beginning  the  great  advance  made  by  as- 
tronomy in  the  last  five  hundred  years. 
Copernicus  dies. 

1546  Tycho  Brahe  born.    The  most  diligent  and 

accurate  observer  since  Hipparchus.  He 
invented  and  made  many  instruments, 
necessarily  crude,  with  which,  however,  he 
did  marvelously  accurate  work.  On  his 
observations  were  founded  Kepler's  laws. 

1564  Galileo   born. 

jgyj  Johann  Kepler  born.     He  assisted  Tycho 

for  several  years,  and  inherited  his  post  at 
the  death  of  the  latter. 

1572  Tycho  observes  a  "new  star,"  the  first 

recorded  since  the  time  of  Hipparchus.  This 
stimulates  him  to  make  a  star  catalogue  of 
great  accuracy.  In  making  this  he  uses 
corrections  of  his  own  for  atmospheric  re- 
fraction. 

1583  Galileo  discovers  isochronism  of  pendulum. 

1600  Bruno  burned  at  the  stake. 

1601  Tycho  dies. 

1605  Brilliant  "new  star"  observed  by  Kepler. 

1608  (?)  First  telescopes  made  in  Holland. 

1609  Galileo  improves  telescope. 

Kepler  publishes  his  work  Astronomia  Nova, 
including  the  first  two  of  his  great  laws  of 
planetary  motion. 

1610  Four  larger  satellites  of  Jupiter,  sun-spots, 
surface  irregularities  of  the  Moon,  phases  of 
Venus,  discovered   by  Galileo  with  his  tele- 
scope.    Saturn's    rings    also   first   seen    by 
Galileo,  but  believed  to  be  illusionary. 

1614  John    Napier   publishes   his   invention   of 

logarithms. 


CHronologfy  191 

Kepler  evolves  his  Third  Law.  1816 

Kepler  dies.  1630 

Transit  of  Venus  foretold  by  Horrocks.  1639 

Galileo  dies.  1642 

Sir  Isaac  Newton  born. 
First  serious  selenographic  work  done  by  1645 

Langrenus. 

John  Flamsteed  born.     First  Astronomer  1646 

Royal.      Maker    of    the    first    trustworthy 

catalogue  of  stars. 

Huyghens  discovers  Titan,  satellite  vi.  of  1655 

Saturn;  the  largest  and  first  to  be  seen. 

Huyghens  describes  in  cipher  the  rings  of  1656 

Saturn. 

Edmund     Halley,      Second     Astronomer, 

Royal  born. 

First  pendulum  clock  made  by  Huyghens.  1657 

Huyghens    translates    cipher    on    Saturn  1659 

rings. 

Newton  experiments  with  prism  and  dis-         1660  (?) 

persion  of  light. 

Reflecting     telescope    first    proposed    by  1663 

Gregory. 

Cassini  notes  great  spot  on  Jupiter.  1665 

Cassini    establishes    rotation    periods    of 

Jupiter. 

Newton  makes  first  reflecting  telescope.  1666 

National  Observatory  at  Paris  founded.  1667 

Telescope  first  used  as  pointer  by  Picard. 
Cassini  discovers  Japetus,  satellite  viii.  of  1671 

Saturn. 

Cassini  discovers  Rhea  satellite  v.  of  Saturn.  1672 

Cassini  discovers  division  in  Saturn's  ring.  1675 

Time  required  by  light  to  travel  discovered 

and  measured  by  Roemer. 

Royal  Observatory  at  Greenwich  founded.  1676 


192    THe  Essence  of  Astronomy 

1681  First  transit  instrument  built  by  Roemer. 

1684  Cassini  discovers  Tethys  and  Dione  satel- 

lites, iii.  and  iv.  of  Saturn. 

1687  Newton  publishes  his  Principia,  containing 

his  exposition  of  the  universal  rule  of  the  law 
of  gravitation. 

1691  Oval  form  of  Jupiter  observed    by    Cas- 
sini. 

1692  Cassini  makes  chart  of  Moon. 

1705  Halley  first  predicts  return  of  a  comet,  the 
one  now  bearing  his  name. 

1706  Solar  prominences  first  noted. 
1707-1713         Miraldi   notes   variation   in   brilliancy   of 

Jupiter's  fourth  satellite. 
1718  Halley  detects  motions  of  Arcturus  and 

Sirius. 
1721  Halley  appointed  the  second  Astronomer 

Royal. 

1727  Newton  dies. 

1729  Bradley   publishes   his   discovery   of   the 

aberration  of  light. 
1738  Herschel  born. 

1742  Halley  dies. 

1743  Heliometer  invented  by  Savary. 

1748  Bradley  discovers  that  the  inclination  of 

the  Earth's  axis  is  not  constant  (nutation}. 

1757  Achromatic  lens  invented  by  Dollond. 

1758  Halley 's  Comet  upon  first  predicted  return, 
first  observed  by  Palitsch. 

1767  Maskelyne,  Astronomer  Royal,  lays  foun- 

dation of  Nautical  Almanac. 
1772  Bode's  "  Law  "  discovered. 

1 775-94  Herschel  establishes  rotation  of  Saturn. 

1781  Herschel  discovers  Uranus. 

1782  Herschel  appointed  Astronomer  to  George 
III. 


Chronology  193 

Goodricke   advances   correct    theory,    ex-  1783 

plaining  variability  of  Algol. 

Laplace  publishes  elements  of  Uranus. 
Herschel  deduces  motion  of  Sun  through 

space. 

Herschel  discovers  two  satellites  of  Uranus.  1787 

Herschel's  great  reflector  finished.  1789 

Herschel  discovers   the   Mimas  and  En- 

celadus  satellites  i.  and  ii.  of  Saturn. 

Large  disks  of  flint-glass  successfully  made         J^QQ  (?) 

by  Guinand. 

Laplace    publishes   Systeme    des    Mondes.  1766 

containing  his  theories  on  the  Nebular  Hy- 
pothesis. 

Piazzi  discovers  first  planetoid  (Ceres).  1801 

De  Zach  rediscovers  the  lost  first  planetoid. 
Olbers  discovers  second  planetoid  (Pallas),  1802 

Herschel   discovers    revolution   of   binary         1803-04 

stars. 

Harding  discovers  third  planetoid  (Juno).  1804 

Olbers  discovers  fourth  planetoid  (Vesta).  1807 

Frauenhofer  first  measures  lines  in  solar  1817 

spectrum. 

Herschel  dies.  1822 

Stower    measures    diameter    of    Jupiter's 

largest  four  satellites. 

Vast  meteoric  shower  in  America.  1833 

Hopkins    Observatory,    Williams   College,  1836 

founded ;  oldest  in  America. 

Chart    of    Moon    made    by    Beer    and  1837 

Maedler. 

Bessel   first   successfully   measures   stellar     1837-1840 

parallax. 

Sun-spot  periodicity  discovered  by  Schwabe  1838 

Pulkowa  Observatory  completed.  1839 

Harvard  College  Observatory  founded. 
13 


194    THe  Essence  of  Astronomy 

1840  Canals  on  Mars  first  indicated  in  drawings 

by  Beer  and  Maedler. 

1842  Doppler    evolves    method    of    measuring 
velocity  in  line  of  sight  of  luminous  body. 

1843  Schwabe  announces  Sun-spot  periodicity. 

1844  Bessel    pronounces    Sirius    and    Procyon 
binary  systems  with  dark  or  very  faint  com- 
panions. 

1845  First  photograph  of  Sun  taken ;  a  daguerreo- 
type by  Foucault  and  Figeau. 

Fifth  planetoid  discovered. 
Lord  Rosse's  great  6  ft.  reflector  built. 
Adams  completes  calculations  of  Neptune's 
position. 

1846  Le  Verrier  completes  computation  of  posi- 
tion of  Neptune. 

1846  Neptune  found  by  Galle. 

Satellite  of  Neptune  discovered  by  Lassel. 

Biela's  comet  observed  to  split. 

1848  Hyperion  satellite  vii.  of  Saturn  discovered 

simultaneously  by  Bond  and  Lassell. 

1850  Bond  successfully  photographs  Moon. 
1851-1860         Solar  prominences  proved  appendages  of 

the  Sun  not  Moon. 

1851  Lamont  connects  variation  of  compass  with 
sun-spot  period. 

Lassell   discovers   two   more   satellites   of 
Uranus. 

1852  "Magnetic  Storms"  found  to  be  periodic. 

1853  De  la  Rue  first  uses  collodion  process  in 
astronomical  photography. 

De  la  Rue  first  uses  stereoscope. 

1857  De  la  Rue  constructs  first  photoheliograph. 

1859  Reversal  of  spectrum  lines  experimentally 

explained  by  Kirchoff. 
1862  Clark  discovers  companion  to  Sirius. 


CHronology  195 

Velocity  of  light  found  experimentally  by 
Fizeau  and  Foucault  independently. 

Rotation  of  Mars  deduced  by  Kaiser. 

Carrington  establishes  the  unequal   rota-  1863 

tion  of  the  solar  surface. 

Nebulae  first  studied  by  spectroscope.  1864 

Donati  first  observes  spectrum  of  comet. 

Spectrum  of  Nebula  first  observed  by 
Huggins. 

Spectrum  of  Sun-spot  compared  with  that  1866 

of  solar  surface  by  Lockyer. 

Return  of  Leonid  meteorites. 

Le  Verrier  computes  elements  of  Leonids.  1867 

Helium  found  in  the  Sun.  1868 

Huggins  measures  velocities  of  stars  in  line 
of  sight. 

Spectroscope  first  used  in  eclipse  of  Sun. 
Solar  prominences  viewed  by  Jansen  for  first 
time  without  eclipse. 

Dawes  makes  first  good  map  of  Mars.  1869 

Gelatine  dry  plates  first  used  in  astronomi-  1876 

cal  photography,  by  Huggins. 

Schiaparelli  first  maps  Martian  "Canals."  1877 

Satellites  of  Mars  discovered  by  Hall. 

Gill  more  correctly  ascertains  distance  of         1877-89 
Sun  by  improved  heliometer. 

Jupiter's    "Great    Red    Spot"    first]  ob-  1878 

served. 

Sir  W.  M.  H.  Christie,  Eighth  Astronomer  1881 

Royal  takes  up  duties. 

Gill   makes  first  comet  photograph,   and  1882 

originates  stellar  chart-photography. 

Great  star  map  started.  1887 

Lick  Observatory  completed.  i88g 

Vogel  observes  orbital  motion  of  Algol  by  1889 

means  of  spectroscope. 


196    XKe  Essence  of  Astronomy 

Spectroscope    reveals    close    binary    stars 
(Pickering). 
1800  Schiaparelli    discovers    true    rotation    of 

Venus  and  Mercury. 

1892  Dr.  Anderson  discovers  "  Nova  Aurigae. " 

Barnard  discovers  Jupiter's  fifth  satellite. 

1894  Sun-spots  proved  not  always  depressions, 
by  Hewlett. 

Lowell  Observatory  founded. 

1895  Constitution  of  Saturn's  rings  proved  by 
spectroscope  (Keeler). 

Helium  discovered  on  the  Earth. 

1896  Companion    to    Procyon    discovered    by 
Schaeberle. 

1897  Yerkes  Observatory  opened. 

1898  Pickering  discovers,  by  photography, 
Phoebe  satellite  ix.  of  Saturn. 

1901  Dr.  Anderson  discovers  "Nova  Persei. " 

1903  Sun  photography  improved  by  Hale  and 
Deslandres  independently. 

1904  Mt.  Wilson  Solar  Observatory  founded. 

1905  Jupiter's  sixth  and  seventh  satellites  dis- 
covered by  photography  at  the  Lick  Observa- 
tory. 

Pickering  discovers,  by  photography,  The- 
mis satellite  x.  of  Saiurn. 

1907  Slipher  of  the  Lowell  Observatory  improves 
plates  for  spectroscope  work. 

Slipher  successfully  photographs  Martian 
Canals. 

1908  Eighth  satellite  of  Jupiter  discovered  by 
Melotte. 

Eros,  the  nearest  planetoid,  discovered  by 
Witte. 


ASTRONOMICAL  SYMBOLS  USED  IN  ALMANACS 


$  =Mercury.         *=New  Moon. 

3=First  Quarter. 
9  =Venus.  O  =Full  Moon. 

(D=Last  Quarter. 
e=Earth.  O=Sun. 

cf  =Conjunction ;  having  the  same 
Right  Ascension.  This 
does  not  necessarily  mean 
at  the  same  point  on  the 
celestial  sphere,  since  the 
Declination  of  the  two 
bodies  may  be  different. 


y=Jupiter. 

^ 

or  $ 


Saturn. 
Uranus. 


<P Opposition;  differing  180°  in 

Right  Ascension 

=Neptune.         D=Quadrature ;  differing  90°  in 
Right  Ascension 


Examples: 

cf  U  $  =Jupiter  and  Venus  in  conjunction. 
cP  cf  O=Mars  in  opposition. 

O=Jupiter  in  conjunction  with  the  Sun. 
§  O  Inf. ^Mercury  in  inferior  conjunction;  be- 
tween the  Earth  and  the  Sun. 
5  O  Sup.=  Mercury  in  superior  conjunction;  on 
the  other  side  of  the  Sun  from  the  Earth. 
$  O=Venus  and  Moon  in  conjunction. 


197 


A  BRIEF  BIBLIOGRAPHY 

The  titles  in  this  list  are  taken  from  only  those  books 
which  the  author  of  this  little  volume  has  been  privileged 
to  read,  and  of  which,  therefore,  he  has  a  personal  knowl- 
edge and  opinion.  The  omission  of  any  title  does  not 
by  any  means  indicate  that  he  has  not  considered  that 
book  worthy  of  mention,  but  merely  that  he  has  not  had 
the  opportunity  to  become  familiar  with  it.  All  really 
technical  works  have  been  left  out,  as  it  is  the  compiler's 
intention  to  suggest  only  books  of  a  more  or  less  "popular" 
nature. 

Books  for  General  Reading 

ARRHENIUS,  SVANTE.  Life  of  the  Universe.    2  vols. 

i6mo.     260  pp.  each. 

A  very  interesting  presentation  of  man's  conception  of  life 
from  the  earliest  times  to  the  present. 

Worlds     in     the      Making. 
I2mo.     230   pp.     Illus. 

A  presentation  of  the  author's  theories  in  clear  everyday 
language;  a  valuable  work  not  only  well  worth  the  reading  but 
most  delightfully  readable. 

BALL,    SIR    ROBERT.  The     Earth's      Beginning. 

I2mo.     384   pp.     Illus. 

A  simple  but  comprehensive  work. 

Star-Land.     I2mo.     402  pp. 

Illus. 

The  author's  lectures  to  young  people,  rewritten.    An  excellent 
book  to  awaken  interest  in  astronomy. 
I98 


A  Brief  Bibliography  199 

BALL,  SIR  ROBERT.  In     Starry     Realms.      8vo. 

350  pp.     Illus. 

A  volume  of  the  same  nature  as  Newcombe's  Side-Lights  on 
Astronomy.  Contains  interesting  chapters  on:  The  Heat  of  the 
Sun;  The  Moon's  History;  A  Visit  to  an  Observatory;  Notes  on 
Nebulae;  Mars  as  a  World;  Extent  of  the  Sidereal  Heavens,  etc. 

The  Story  of  the  Heavens. 
8vo.  568  pp.  Fully 
Illus. 

This  author's  largest  and  most  comprehensive  popular  work. 
His  ultra-conservative  attitude  toward  theories  not  yet  wholly 
substantiated  is  somewhat  to  be  deplored. 

BICKERTON,  A.  W.  The  Birth  of  Worlds  and 

Systems.     i6mo. 
163  pp.     Illus. 

A  most  interesting  book  on  cosmic  evolution.  It  presents  a 
special  and  original  theory  regarding  stellar  collisions.  Technical 
in  parts. 

BLACK,  F.  A.  Natural  Phenomena.    8vo. 

366  pp.     Illus. 

Contains  interesting  chapters  on:  The  Wandering  of  the  Poles; 
The  Weather;  The  Zodiacal  Light;  The  Turn  of  the  Day,  etc. 

Problems  in  Time  and  Space. 

8vo.     361  pp.     Illus. 

A  companion  volume  to  the  above.  Contains  chapters  on: 
Time;  The  Calendar;  Measuring  the  Earth;  Magnetism  of  the 
Earth;  Movement  of  the  Earth  and  Sun  in  Space,  etc. 

BURGEL,   BRUNO   H.  Astronomy  for  AIL     8vo 

351  pp.        Profusely 
Illus. 

A  large  book  containing  much  "gossipy"  information  not 
found  in  other  volumes.  Its  illustrations  are,  in  many  instances, 
unusual. 

CHAMBERS,  GEORGE  F.  Astronomy.     i6mo.    335  pp. 

Profusely  Illus. 

A  thorough  little  volume  of  convenient  size.  Noteworthy 
particularly  for  its  illustrations. 


2OO   THe  Essence  of  Astronomy 

CHAMBERS,  GEORGE  F.          The  Story  of  Eclipses.    i6mo. 

208  pp.     Illus. 

A  good  rdsum6  of  this  division  of  astronomical  history. 

CLERKE,   AGNES   M.  Problems  in  Astrophysics. 

8vo.     567    pp.     Illus. 

A  rather  technical  and  special  work  on  the  physical  constitu- 
tion of  the  Sun  and  stars.  Despite  its  somewhat  advanced 
style  it  contains  much  within  the  easy  comprehension  of  the 
general  reader. 

The    System    of    the    Stars. 

8vo.     403  pp.     Illus. 

Of  the  same  general  nature  as  the  former  volume,  but  not  so 
special.  Has  interesting  chapters  on  the  unusual  stars,  the 
Variables  and  Novae. 

FLAMMARION,    CAMILLE.          Popular   Astronomy.    8vo. 

696  pp.    Profusely  Illus. 

Most  fascinating  reading.  The  book  is  exhaustive,  and  the 
author  has  allowed  his  imagination  full  sway.  For  this  rea- 
son it  must  be  read  somewhat  sceptically.  Too  many  state- 
ments of  facts  and  figures  require  checking,  as  the  book  is  not 
quite  up-to-date. 

FORBES,  GEORGE.  History  of  Astronomy 

i6mo.     200   pp.     Illus. 

A  capital  summary,  presenting  its  subject  in  well-classified 
and  readable  form.  The  best  short  history  of  astronomy. 

JACOBY,    HAROLD.  Astronomy,    a    Popular 

Handbook.     8vo.     435 
pp.     Well  Illus. 

A  recent  book  by  a  conservative  with  a  "stand-pat"  attitude. 
It  is  divided  conveniently  into  two  parts,  the  first  for  the  general 
reader,  the  second  for  the  student. 

KIPPAX,  JOHN  R.  The  Call  of  the  Stars.     8vo. 

43°  PP-     Profusely  Illus. 

"A  popular  introduction  to  a  knowledge  of  the  starry  skies.  " 
The  author  has  successfully  blended  the  facts  of  astronomy  with 
the  legendary  lore   of  the   stars  and  constellations.     A   most 
..    readable  and  interesting  work. 


A  Brief  Bibliography          201 

LOWELL,    PERCIVAL.  The    Evolution    of    Worlds. 

8vo.    262  pp.    Illus. 

A  clear  and  logical  presentation  of  a  theory  opposed  to  the 
"Nebula  Hypothesis"  by  the  foremost  progressive  in  the  astro- 
nomical world. 

Mars  as  the  Abode  of  Life. 
8vo.     288    pp.    Illus. 

A  summary  of  Professor  Lowell's  views  on  evolution,  with 
especial  reference  to  Mars.  Well  worth  the  reading. 

Mars.          8vo.          Illus. 

Mars    and     Its     Canals. 

8  vo.  Illus. 

Volumes  presenting  Professor  Lowell's  descriptions  of  Mars 
and  his  theories  concerning  it;  also  reporting  in  untechnical 
language  the  result  of  his  long  study  of  the  planet. 

The  Solar  System.     I2mo. 

1 34  pp.     Diagrams. 

A  small  technical  volume  containing,  however,  much  that  is 
readily  understood  by  the  general  reader.  It  offers  several 
interesting  theories. 

LYNN,  WILLIAM  T.  Remarkable  Comets.    321110. 

75  pp.     Illus. 

A  little  book  giving  briefly  the  histories  of  the  more  famous 
comets. 

MACPHERSON  JR.,   HECTOR.      The  Romance  of  Modern 

Astronomy.     I2mo. 
330  pp.     Illus. 

The  title  is  somewhat  misleading.  It  is  a  good  general  book 
on  astronomy.  Contains  a  brief  history  of  astronomical  dis- 
coveries. 

MORSE,  EDWARD  S.  Mars  and  Its  Mystery.     8vo. 

200  pp.     Illus. 

A  study  of  the  planet  for  the  general  reader,  by  a  strong  sup- 
porter of  Prof.  Lowell. 

NEWCOMBE,    SIMON.  Astronomy  for   Everybody. 

8vo.     333  pp.     Illus. 

A  concise  exposition  of  astronomical  facts. 


2O2    THe  Essence  of  Astronomy 

NEWCOMBE,  SIMON.  Side-Lights  on  Astronomy. 

8vo-     345  PP-     Hlus. 

A  collection  of  interesting  magazine  articles  on  astronomy  and 
kindred  subjects.  Contains  chapters  on:  Unsolved  Problems; 
Extent  of  the  Universe;  Can  We  Make  It  Rain;  Life  in  the 
Universe;  How  the  Planets  are  Weighed,  etc. 

The  Stars,  a  Study  of  the 
Universe.  8vo.  333  pp. 
Illus. 

A  thorough  treatment  of  the  subject.  Somewhat  technical, 
but  not  at  all  beyond  the  layman. 

OLCOTT,  WM.  TYLER.  Star  Lore  of  All  Ages.    8vo. 

45°       PP-        Profusely 
Illus. 

A  splendid  collection  of  all  the  myths  of  the  world  relating  to 
the  stars. 

Sun  Lore  of  All  Ages.     8vo. 
45°       PP-     Profusely 
Illus. 
A  companion  volume  to  the  above,  treating  only  of  the  Sun. 

POOR,  CHARLES  LANE.  The  Solar  System,  a  Study 

of  Recent    Observations. 
8vo.     309  pp.     Illus. 

A  scholarly,  but  untechnical  book  by  a  rigidly  conservative 
astronomer. 

SERVISS,  GARRETT  P.  Astronomy  in  a  Nutshell. 

I2mo.     261    pp.     Illus. 

A  brief  summary.  Devotes  much  of  its  space  to  describing 
the  Celestial  Sphere  and  its  coordinates.  Time  and  the  calendar 
are  well  explained. 

Curiosities  of  the  Sky.     8vo. 

267  pp.     Illus. 

Fascinating  reading.  Contains  chapters  on:  The  Windows  of 
Absolute  Night;  The  Wrecking  of  the  Moon;  The  Passing  of  the 
Constellations;  The  Strange  Adventures  of  Comets,  etc. 


A  Brief  Bibliography  203 

SNYDER,  CARL.  The  World  Machine.     8vo. 

490  pp. 

One  of  the  most  readable  and  interesting  presentations  of  the 
growth  of  astronomical  knowledge.  Well  worth  the  reading. 

TODD,  DAVID.  Stars  and  Telescopes.     I2mo. 

419  pp.     Illus. 

A  good  volume  which  needs  reprinting.  The  later  revisions  are 
in  the  form  of  long  footnotes,  and  consecutive  reading  is  almost 
impossible. 

TURNER,     HERBERT    H.  Astronomical    Discovery. 

8vo.     225  pp.     Illus. 

Well  told  stories  of  the  important  astronomical  discoveries; 
Uranus,  Neptune,  The  Asteroids,  etc. 

WILLIAMS,  HENRY  S.  Miracles  of  Science.    8vo. 

343  pp.     Illus. 

Popular  magazine  articles.  Good  reading  but  inaccurate  in 
several  instances.  Only  the  first  100  pages  are  devoted  to 
astronomical  subjects. 

Books  for  Observers 

BAIKIE,  JAMES.  Through  the  Telescope.     8vo. 

291  pp.     Illus. 

A  useful  practical  handbook  on  telescope  work  for  the  amateur. 

Ephemeris  and  Nautical  Almanac,  The  American.     8vo. 
Diagrams. 

Published  annually  by  the  Government.  Indispensable  to  the 
observer.  It  gives  all  figures  regarding  the  positions  of  the 
celestial  bodies,  including  the  satellites  of  the  planets.  All 
predictable  phenomena,  such  as  eclipses,  transits,  occultations, 
etc.,  are  fully  covered. 

GIBSON,  FRANK  M.  The  Amateur  Telescopists 

Handbook.     121110. 
163  pp.     Illus. 

A  practical  handbook  on  the  testing,  the  care  and  the  use  of 
the  telescope.  Contains  an  observer's  catalog  of  celestial  objects 
visible  in  small  instruments. 


204    THe  Essence  of  Astronomy 

HASLUCK,  PAUL  N.  Telescope  Making.     i2mo. 

1 60  pp.     Diagrams. 
A  manual  for  those  desiring  to  construct  their  own  instrument. 

McKREADY,    KELVIN.  A     Beginner's    Star-Book. 

4to.     148    pp.     Fully 
Illus. 

By  far  the  best  volume  for  the  observer  beginning  work, 
whether  with  the  naked  eye,  field-glass,  or  telescope.  It  contains 
a  system  of  star-maps  and  charts  arranged  on  an  original  and 
most  convenient  plan.  The  text  is  extremely  well  written,  and  is 
supplemented  by  an  admirable  observer's  catalog  of  objects 
within  the  reach  of  the  small  instrument. 

MARTIN,  MARTHA  EVANS.       The  Friendly  Stars.     I2mo. 

260  pp.     Diagrams. 
A  simple  hand-book  for  the  naked-eye  observer. 

The    Way    of    the    Planets. 
I2mo.     275  pp.     Illus. 

A  companion  book  to  the  above,  devoting  itself  to  the  planets 
alone. 

OLCOTT,  WM.  TYLER.     A  Field  Book  of  the  Stars.     i6mo. 

163  pp.     Charts. 

The  most  convenient,  and  the  best  small  book  for  the  naked- 
eye  observer.  To  each  constellation  is  devoted  a  special  chart, 
with  a  full  description  on  the  page  facing  it.  Tables  of  all  the 
well-known  meteor-showers  are  given. 

In  Star-Land  with  a  Three- 
Inch  Telescope.  i6mo. 
146  pp.  Charts. 

A  companion  volume  to  the  Field  Book.  Not  as  full  as  the 
Beginner's  Star  Book,  but  the  best  small  volume  for  its  purpose . 

PROCTOR,  MARY.  Half-hours  with  the  Summer 

Stars.     i6mo.     230  pp. 
Illus. 

A  reprint  of  some  charming  newspaper  articles  which  appeared 
a  few  years  ago.  The  volume  contains  in  its  introduction  a  fine 
description  of  the  Yerkes  observatory. 


A  Brief  Bibliography  205 

PROCTOR,   R.   A.  Easy  Star  Lessons.    8vo. 

240  pp.     Illus. 

A  standard  book,  well  arranged. 

Half-Hours   with   the   Stars. 

4to.     pp.     1 2  maps. 
An  easy  guide  to  the  constellations. 

Books  on  the  Moon 

FAUTH,    PHILIP.  The  Moon  in  Modern  As- 

tronomy. 8vo.  i6opp. 
Illus. 

A  summary  of  twenty  years '  selenographic  work,  and  a  study 
of  recent  problems. 

Not  only  excellent  special  reading,  but  most  useful  to  the 
observer.  Contains  many  detailed  charts,  drawn  by  the  author. 

MOREAUX,  THE  ABBE  A  Day  in  the  Moon.     i2mo. 

199  pp.     Illus. 

A  special  volume  in  untechnical  language. 

NASMYTH  AND  CARPENTER       The  Moon,  Considered  as  a 

Planet,  A  World,  and  a 
Satellite.  I2mo.  315 
pp.  Illus. 

One  of  the  most  interesting  books  on  the  Moon.  It  devotes 
much  space  to  the  exposition  in  simple  language  of  the  authors' 
theories  regarding  the  origin  of  the  lunar  surface  markings. 

SERVISS,    GARRETT    P.  The     Moon,     a     Popular 

Treatise.  I2mo.  248 
pp.  Illus. 

A  very  readable  book  in  the  form  of  conversations.  Contains 
a  fine  series  of  photographs. 

RUDAUX,    L.  How    to    Study    the    Stars. 

I2mo.     360   pp.     Illus. 

Instructions  on  the  use  of  the  telescope,  and  much  very 
valuable  information  on  astronomical  photography  for  amateurs. 
A  full  chapter  is  devoted  to  naked-eye  observing. 


206    TKe  Essence  of  Astronomy 

WEBB,  REV.,  T.  W.  Celestial  Objects  for  Common 

Telescopes.     2  vols.     12 
mo.     233  and  280  pp. 

Volume  I  is  restricted  to  the  solar  system,  vol.  n  covering  the 
rest  of  the  universe.  By  far  the  most  complete  observer's 
catalog  of  its  class.  It  has  been  standard  for  years,  and  is 
invaluable  for  its  purpose. 


Text  Books 


HOLDEN,     EDWARD    S. 


NEWCOMB,  SIMON 
AND  HOLDEN,  E.  S. 


TODD,  DAVID. 
YOUNG,  CHARLES  A. 


Elementary        Astronomy. 
I2mo.     446   pp.     Illus. 


Astronomy,  for  High-Schools 
and  Colleges.  8vo.  501 
pp.  Illus. 

A  New  Astronomy.  I2mo. 
480  pp.  Illus. 

The  Elements  of  Astronomy. 
I2mo.  438  pp.  Illus. 


General    Astronomy. 
600  pp.     Illus. 


8vo. 


Atlases 

PECK,  W.  The  Observer's  Atlas  of  the 

Heavens.       Folio.       30 
Maps. 

Contains  several  catalogs  of  double  stars,  variable  stars, 
clusters,  etc.  There  are  several  special  plates  of  much  value.  A 
capital  chart  of  the  Moon  is  also  included.  The  maps  and  charts 
are  supplemented  by  a  well-written  text  containing  many  most 
convenient  and  useful  tables.  It  is  the  best  atlas  for  the  user  of 
a  small  instrument. 


A  Brief  Bibliography          207 

PROCTOR,  R.  A.  A  New  Star  Atlas.     ;th  Ed. 

Folio.     12     Maps    and 
Index  Maps. 

For  long  the  standard  atlas,  and  by  many  still  considered  the 
best. 

SCHURIG,  RICHARD.  Tabulae  Celestes  (Himmel's 

Atlas}.    Folio   8   Maps 
and  Moon-Chart. 

The  best  cheap  star-atlas.     The  text  is  in  Latin  and  German, 
but  there  is  not  enough  of  it  to  make  a  translation  necessary. 

BARRETT,  LEON 

AND  SERVISS,  GARRETT  P.          Star  and  Planet  Finder. 

The  only  good  planisphere,  and  the  only  one  by  which  the  ^ 

positions  of  the  planets,  the  Sun,  and  the  Moon  can  be  found  for  ' 

every  day  of  the  year.  It  is  convenient  and  easy  to  use,  and, 
until  a  general  knowledge  of  the  constellations  is  gained,  of  great 
assistance  to  the  observer. 


A   Revised   Edition   of    a   Standard  Work 


A  Field  Book  of  the 
Stars 

By  William  Tyler  Olcott 

Author  of  "Star  Lore  of  All  Ages,"  etc. 


Second  Edition,  Revised.      With  Additional  Material 
16°.      With  Fifty  Diagrams.     $1.00 

In  the  new  edition  of  this  standard  work 
the  author  has  given  full  information,  together 
with  diagrams  representing  the  result  of 
the  latest  investigations.  All  matters  of  a 
technical  or  theoretical  nature  have  been 
omitted.  Only  what  the  reader  can  observe 
with  the  naked  eye  or  with  an  opera-glass 
have  been  included  in  it. 

Excellently  arranged  and  copiously  illus- 
trated, this  manual  is  a  real  field-book  and 
will  prove  valuable  for  all  who  wish  to  become 
familiar  with  the  stars. 

New  York         G.  P.  Putnam's    Sons         London 


The  Solar  System 

A  Study  of  Recent  Observations 

By  Prof.  Charles  Lane  Poor 

Professor  of  Astronomy  in  Columbia  University 

8°.     With  Frontispiece  in  Photogravure,  6  Plates,  and  32 

Cuts  and  Figures.     No.  18 >  Science  Series.     Net,  $2.00 

(By  mail,  $2,20) 

The  subject  is  presented  in  untechnical  language  and 
without  the  use  of  mathematics.  Professor  Poor  shows 
by  what  steps  the  precise  knowledge  of  to-day  has  been 
reached  and  explains  the  marvellous  results  of  modern 
methods  and  m  dern  observations. 

History  of  Astronomy 

By  George  Forbes,  M.A.,  F.R.S.,  M.Inst.C.E. 

Formerly  Professor  of  Natural  Philosophy, 
Anderson's  College,  Glasgow 

16°.     Adequately  Illustrated.     Net  75  cents 

(By  mail,  85  cents) 
No,  .1    A  History  of  the  Science  Series 

The  author  traces  the  evolution  of  intellectual  thought 
in  the  progress  of  astronomical  discovery,  rscognizing  the 
various  points  of  view  of  the  different  ages,  giving  due 
credit  even  to  the  ancients.  It  has  been  necessary  to 
curtail  many  parts  of  the  history,  to  lay  before  the  reader 
in  a  limited  space  enough  about  each  age  to  illustrate  its 
tone  and  spirit,  the  ideals  of  the  workers,  the  gradual 
addition  of  new  points  of  view  and  of  new  means  of  in- 
vestigation. 

The  volume  is  divided  as  follows  t 

The    Geometrical    Period — The    Dynamical  Period- 
Observation.    The  Physical  Period. 


New  York         G.  P.  Putnam's    Sons          London 


Star  Lore  of  All  Ages 

A  Collection  of  Myths,  Legends,  and  Facts, 

Concerning  the  Constellations  of  the 

Northern  Hemisphere 

By  William  Tyler  Olcott 

Author  of  "A  Field  Book  of  the  Stars,"  etc. 

8°.     With  164  Illustrations  and  Diagrams 
$3.50  net.     By  mail  $3.75 

"A  sumptuous  treasure-house  in  which  the 
lover  of  ancient  myth  and  modern  miracle  may 
wander  for  enchanted  hours.  Its  red  and  gold 
doors  open  upon  a  gallery  of  masterpieces  in  art, 
all  being  inspired  conceptions  connected  in  some 
way  with  star  lore.  The  pictures  and  the  legends 
of  all  ages  have  been  ransacked  to  make  this 
gallery  irresistible  to  art  and  nature  lovers,  and 
not  in  vain.  Withal  the  most  intense  facts  of 
astronomical  knowledge  and  discovery  are  given 
beside  the  tapestried  romance  which  earlier 
peoples  wove  with  star-stuff  as  the  medium  of 
their  art.  It  is  a  book  delightful  to  the  eye, 
scholarly  in  conception  and  treatment,  and 
charming  to  the  inner  sense." — From  Scientific 
American, 

"  Here  all  the  myths,  legends,  and  facts  relat- 
ing to  all  our  northern  constellations  are  set  forth 
fully  as  well  as  with  scientific  exactness,  are 
illustrated  with  clear  maps  and  diagrams,  and 
are  brought  into  close  association  with  their 
classic  origins  by  beautifully  reproduced  photo- 
graphs of  works  of  art  and  architecture." 

The  Outlook. 


New  York         G.  P.  Putnam's    Sons         London 


Beginner's  Star=Book 

An  Easy  Guide  to  the  Stars  and  to  the  Astro- 
nomical Uses  of  the  Opera-Glass,  the 
Field-Glass,  and  the  Telescope 

By  Kelvin  McKready 

Square  8vo.     Including  70  Illustrations.     $2.50  net 
By  mail  $2.75 

This  volume,  peculiarly  definite  and  helpful  in  method, 
is  especially  adapted  to  the  practical  needs  of  those  who 
wish  a  well  illustrated  and  clearly  written  handbook, — 
accurate  in  its  scientific  information  and  yet  popular 
enough  to  meet  the  wishes  of  the  average  man  or  woman. 
The  book  is  distinguished  from  other  volumes  on  popular 
astronomy  by  a  novel  system  of  mapping,  and  by  an  un- 
usually full  discussion  of  the  uses  of  the  simpler  astro- 
nomical instruments.  Special  care  has  been  given  to 
reproductions  of  recent  astronomical  photographs. 

"Mr,  McKready's  book  is  the  finest  thing  for  the  beginner 
that  I  have  ever  seen.  It  is  a  pleasure  to  read  a  book  so  ex- 
cellent in  so  many  different  ways,  I  have  never  seen  photo* 
graphs  reproduced  better,  I  have  never  seen  maps  of  the 
heavens  so  intelligible  to  the  beginner,  and  I  have  never  read 
explanations  so  concise  and  yet  so  complete  as  in  this  book" 
— Prof.  S.  A.  Mitchell,  Department  of  Astronomy,  Columbia 
University,  New  York  City. 

New  York         G.    P.    Putnam's    Sons         London 


UNIVERSITY  -OF  CALIFORNIA 


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12   J933 
SI  1933 

S     1933 


M&K    101939 


SE?  15  1940 


MAR  31 1941M 
MAY    17 

ABB1  41954  U> 


20Apr'56BC 
APR  9 -1956  tflf 


m 
. 


CD  LD 

OCT  i  8  1362 


LD  21-50m-l,'33 


YB   17086 


net 


292213 


9 


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